CN113767067A

CN113767067A - Method for treating residue of physical enrichment of bauxite

Info

Publication number: CN113767067A
Application number: CN202080032544.XA
Authority: CN
Inventors: 伊夫·奥切洛
Original assignee: Ib2 Co
Current assignee: Ib2 Co
Priority date: 2019-04-29
Filing date: 2020-04-22
Publication date: 2021-12-07
Also published as: FR3095433A1; WO2020221968A1; AU2020265794A1

Abstract

A process for the manufacture of alumina trihydrate or alumina from a physically enriched residue of bauxite, which has been pretreated by a process comprising calcination (2020) and leaching (2030) to obtain a residue referred to as pretreated residue, said process comprising the steps of: dissolving (2120) the pre-treated residue in an aqueous sodium hydroxide solution having a concentration of 100g Na at a temperature of at least 100 ℃₂O/L and 220gNa₂Between O/L; separating (2130) the solid residue from the liquid phase; crystallizing (2160) aluminum trihydrate by adding seed crystals (2170); separating (2180) the crystalline aluminum trihydrate from the liquid phase, optionally calcining (2194) the aluminum trihydrate to obtain alumina (2196).

Description

Method for treating residue of physical enrichment of bauxite

Technical Field

The present invention relates to the field of mineral processing, and in particular to the physical and chemical processing of bauxite. The invention particularly relates to the recovery of bauxite physical enrichment residue. Physical enrichment of bauxite is understood to mean in particular washing screening and flotation processes which are used industrially to separate finely divided bauxite from certain fractions which cannot be recovered by the bayer process. These unrecoverable fractions still contain aluminum but cannot be extracted by economically viable methods.

The invention relates to a method for treating these bauxite physically enriched residues, which makes it possible to extract part of the aluminium contained in these residues, which can be treated chemically. In the process, the residue is first preheated, followed by leaching, and the pretreated residue is then used in the bayer process to extract aluminum in the form of aluminum trihydrate, which can be converted to alumina.

Background

Aluminum is the third most abundant element in the earth's crust, second only to oxygen and silicon. It is associated with oxygen and is present in a large number of rocks. The major industrial ore of aluminum is bauxite, discovered by geologist Pierre Berthier in 1821 in Les Baux village (france). Bauxite is a complex mixture of oxides of aluminum, iron and silicon, which may contain various impurities such as titanium, calcium, magnesium. More specifically, bauxite is an ore composed mainly of three aluminum minerals, i.e., gibbsite ((Al (OH))₃) Boehmite (γ -AlO (OH)), and diaspore (AlO (OH)), with a small amount of iron minerals, i.e., goethite and hematite (Fe)₂O₃) (imparting a characteristic colour to bauxite), and aluminosilicates (kaolinite, illite … …) and titanium minerals (anatase and rutile (TiO)₂) (ii) a Ilmenite (FeTiO)₃). Some bauxites also contain varying amounts of muscovite (K)_1-x,Na_x)(Al_2-yFe_y)(AlSi₃)(O₁₀(OH)₂) Kaolinite (Al)₂Si₂O₅(OH)₄) Quartz (SiO)₂) Siderite (FeCO)₃) Goethite (FeO (OH)) and magnetite (Fe)₃O₄)。

The main industrial process for extracting aluminium (in the form of oxides) from bauxite is the bayer process, which was developed at the end of the 19 th century. It basically comprises two steps: the first step is the leaching of the ore under pressure with a sodium hydroxide solution (see DE 43977 patent 8/3/1888), and the second step is the precipitation of pure hydrated alumina from the sodium aluminate solution obtained in this way by seeding with hydrated alumina crystals (see DE65604 patent 11/892/3). This precipitated hydrated alumina may then be heat treated to dehydrate it; this thermal treatment also determines the structure and morphology of the alumina obtained, taking into account its use (production of aluminium by molten-salt electrochemical reduction of alumina in the hall-heroult process, or as industrial alumina, in particular in the ceramics industry).

More precisely, the bayer process essentially consists of the selective dissolution (digestion) of the alumina hydrates contained in the bauxite with a hot caustic soda solution (called "liquor"), which is recycled. After the bauxite residue (said residue is called "red mud") has been separated by decantation and washing, the sodium hydroxide solution rich in sodium aluminate is decomposed after cooling (crystallization phase) for the precipitation and extraction of alumina trihydrate (Al)₂O₃-3H₂O); the alumina trihydrate is then washed and calcined at high temperature to produce alumina (Al)₂O₃). The liquor that consumes sodium aluminate after the crystallization stage and is diluted by the addition of water mainly from the washed bauxite residue is evaporated and recycled for processing.

The composition of bauxite depends on its geographical origin. This change in composition is related to the content of the main elements (Al, O, Si), the content of impurities and their mineral structure. For example, laterite-type bauxite (on aluminosilicate geological substrates) from mines located in guinea or australia typically have a relatively high gibbsite content (which is a trihydrate), while the silicon content is lower than that of karst-type bauxite (on carbonate geological substrates) from some mines located in iran, hassakestan, aberray and turkey, where the aluminium is predominantly present as boehmite and diaspore (two variants of the monohydrate). Thus, one simple parameter representing the quality of bauxite is the ratio of alumina to silica, abbreviated as "a/S ratio". For example Al from bauxite of Guinea₂O₃/SiO₂Is generally about 20 or more, fromThe ratio of bauxite in west australia is higher than 15. In bauxite from north queensland, aluminum is mainly present in the form of boehmite and gibbsite.

However, the content of aluminium is not the only criterion: the aluminum must be in a chemical and crystalline form that can be extracted from bauxite by the bayer process. It is known that the conventional bayer process cannot dissolve aluminum in aluminosilicates: this aluminum is lost in a residue called "red mud". In addition, it is known that aluminosilicates contained in red mud carry away part of the sodium hydroxide, thereby increasing the overall consumption of sodium hydroxide in the bayer process; this is described in Xiaofeng Zhu et al in Light Metals, 2013, page 239-244(TMS), "Basic Research on Calcification Transformation of Low Grade Bauxite", Process of Low Grade Bauxite ".

For some types of bauxite deposits, the bauxite needs to be enriched to obtain an ore suitable for the bayer process. These enrichment methods are usually physical methods. Bauxite is known to be a heterogeneous complex mineral that may separate during fine comminution, and the purpose of the bauxite physical beneficiation process is to separate silicon-rich particles (in different mineral forms) from aluminum-rich particles (in different mineral forms).

The most common physical method for such enrichment is wash screening. For example, in the case of bauxite extracted in the northeast Australia, Brazil and south China, washing and screening are widely used. Another physical method is flotation; the method is less in use and is mainly used in the north of China. Another physical process for enriching bauxite is gravimetric separation, which exploits density differences between particles of different compositions. Another physical method is size separation, which takes advantage of the fact that certain compounds are preferably found in the fines after grinding the bauxite.

Each of these physical enrichment processes is intended to separate bauxite into at least two fractions: a fraction that can be recovered in the bayer process, and a fraction that is not recoverable in the bayer process. The latter fraction constitutes a residue (these residues are called "tailings"). It includes not only silicon in various mineral forms but also aluminum, but this aluminum cannot be extracted in a viable economic manner by any known industrial process. Thus, according to the prior art, these residues are the final waste that must be disposed of in a landfill; they are added to the residue of the bayer process (known as "red mud"), which also has not a sufficient capacity of recovery options and in most cases must also be deposited in landfills. In this manner, industrial sites used to extract and utilize bauxite can generate large amounts of residue.

By way of example and giving an order of magnitude concept, the production of one ton of alumina from bauxite, which is typically enriched by physical means, typically requires about 5 tons of bauxite, of which about half can be recovered in the bayer process, the other half forming a physically enriched residue of bauxite. Of the approximately 2.5 tons of enriched bauxite produced from the bauxite physical enrichment process, the bayer process produces approximately 1 ton of alumina and approximately 1 ton of red mud, with the remainder being lost in gaseous form primarily during the calcination of aluminum trihydrate to alumina.

The present invention aims to propose a process for treating physically enriched residues of bauxite, the purpose of which is to extract at least part of the aluminium contained in these residues.

Disclosure of Invention

According to the invention, this problem is solved by a process for treating physically enriched residues of bauxite ores, which comprises a pretreatment of said residues, which improves their suitability for use as raw material in the known bayer process, and adapts the bayer process to such pretreated residues. In particular, this pretreatment of the residues increases the dissolution rate of the aluminium contained in these residues in the modified bayer process.

The method according to the invention comprises two sequential steps. In a first sequence of steps, referred to herein as "pretreatment", the physically enriched residue of bauxite ore is treated by thermal and/or chemical means for the chemical conversion of said residue. The intermediate product resulting from this first sequence of steps is referred to herein as "pre-treatment residue". This pretreatment enhances the applicability of the physically enriched residue of bauxite for use as a base material in the bayer process.

In a second sequence of steps, the pre-treatment residue is introduced into the bayer process. The bayer process has been known in this way since the end of the 19 th century. However, the inventors have found it useful to apply the bayer process specifically to this pre-treated residue.

Said pre-treatment of the physically enriched residue of bauxite comprises a first pre-treatment step, which is a physical pre-treatment, i.e. a thermal treatment. The thermal pre-treatment (calcination) is intended to cause chemical and crystallographic modifications of the residue (or of at least some of the mineral phases constituting the residue). Advantageously, the crushed residue is subjected to this first pre-treatment step of a physically enriched residue of bauxite. The first pretreatment step produces a modified residue, which is then subjected to a second chemical pretreatment step.

More specifically, the thermal pretreatment is carried out at a temperature and for a time such that at least a portion of the silicates present in the physically enriched residue of bauxite are converted to amorphous silica. This temperature is somewhat dependent on the source and nature of the initial bauxite, and is typically about 960 f

Between c and about 1050 c.

In most cases, the first step of the pretreatment process is carried out at a temperature between 980 ℃ and 1060 ℃, preferably between 1000 ℃ and 1050 ℃. The duration of the calcination in this temperature range can be between 15 minutes and 60 minutes, preferably between 20 minutes and 45 minutes. In an advantageous embodiment, the calcination is carried out at a temperature between 1015 ℃ and 1045 ℃ for a period of time between 20 minutes and 45 minutes. For example, calcination is carried out at 1030 ℃ for 30 minutes. The calcination step typically includes a temperature ramp and a cooling ramp; the duration of the display is the dwell time in the specified temperature zone. During this calcination, the layer silicate is dehydroxylated and converted. The water produced during the reaction leaves in the gas phase. For example, it is generally at a temperature between about 530 ℃ and about 590 ℃ in accordance withReaction scheme 1Dehydroxylating kaolinite:

Al₂O₃ x 2SiO₂ x 2H₂o (kaolinite) → Al₂O₃ x 2SiO₂(metakaolin) +2H₂O)

Reaction scheme 1

Also, gibbsite, boehmite, diaspore and other mineral forms that may be present in the residue are dehydroxylated.

According to reaction scheme 2 or 3, metakaolin is gradually transformed to form an amorphous silica phase at a temperature between about 900 ℃ and about 1100 ℃; above about 1030 ℃, the appearance of mullite is observed.

Al₂O₃x2SiO₂(metakaolin) → gamma-Al₂O₃+2SiO₂(amorphous)

γ-Al₂O₃+2SiO₂(amorphous) → 1/3(3 Al)₂O₃x2SiO₂)+4/3SiO₂

Reaction scheme 2

Al₂O₃ x 2SiO₂(metakaolin) → Al₂SiO₅+SiO₂(amorphous)

Al₂SiO₅+SiO₂(amorphous) → 1/3(3 Al)₂O₃ x 2SiO₂)+4/3SiO₂

Reaction scheme 3

In the process according to the invention, the formation of mullite is not desired, since this phase is insoluble under the preferred leaching conditions, as will be explained below.

During this calcination step, the loss of mass of the residue is mainly related to the loss of water, which is caused in particular by the dehydroxylation of phyllosilicates, gibbsite, boehmite and diaspore. The mass loss is usually around 10% to 20% or 11% to 18%.

The first object of the present invention is therefore a process for the production of alumina trihydrate or alumina from bauxite physically enriched residue, which has been pretreated by a process comprising calcination and leaching, so as to obtain a so-called pretreated residue,

the method comprises the following steps:

(a) treating (referred to as leaching) the pre-treated residue with an aqueous sodium hydroxide solution having a concentration of 100g Na at a temperature of at least 100 ℃₂O/L and 220g Na₂Between O/L, preferably 140g Na₂O/L and 200g Na₂Between O/L, more preferably 155g Na₂O/L and 190g Na₂Between O/L, and even more preferably 160g Na₂O/L and 180g Na₂Between O/L;

(b) separating the solid residue from the liquid phase;

(c) crystallizing the aluminum trihydrate by adding seed crystals;

(d) separating the crystallized aluminum trihydrate from the liquid phase,

(e) optionally, calcining the aluminum trihydrate obtained in step (d) to obtain alumina.

Advantageously, the temperature of step (a) is between 150 ℃ and 350 ℃, preferably between 200 ℃ and 300 ℃, more preferably between 220 ℃ and 280 ℃, and even more preferably between 250 ℃ and 270 ℃.

Advantageously, Al of the pretreated residue₂O₃/SiO₂The mass ratio is greater than 3.0, preferably greater than 4.0, and even more preferably greater than 5.0.

Advantageously, the alumina content of the pre-treatment residue is greater than 50% by mass, preferably greater than 55% by mass and even more preferably greater than 60% by mass. Advantageously, the pretreated residue has a silica content of less than 20% by mass, preferably less than 15% by mass, and even more preferably less than 14% by mass. Advantageously, the loss on ignition of the pre-treatment residue is less than 5%, preferably less than 3%, and even more preferably less than 2.5%. The diaspore content of the pretreatment residue is less than 3 mass%, and the kaolinite content of the pretreatment residue is less than 3 mass%.

In an advantageous embodiment of the process, the liquid phase from step (d) is reintroduced into the aqueous sodium hydroxide solution used in step (a).

Said pre-treatment comprises a first calcination step, advantageously carried out at a temperature between 980 ℃ and 1050 ℃, preferably between 990 ℃ and 1040 ℃, and even more preferably between 1000 ℃ and 1040 ℃. The pretreatment comprises a second leaching step using aqueous sodium hydroxide solution.

The thermal pretreatment process may be used at least in part at an industrial production site for bauxite ore, or entirely at an industrial site where a bayer process is deployed. It requires special equipment, i.e. a kiln. Advantageously, the method is carried out on comminuted residues.

According to the invention, the process for treating a physically enriched residue of bauxite comprises a second pretreatment step, which is a chemical step. The second pretreatment step comprises leaching the calcined residue with sodium hydroxide. The leaching is carried out using a solution called "leachate", which is an alkaline solution, and preferably a sodium hydroxide solution. In this step, under appropriate conditions of temperature, residence time, sodium hydroxide concentration and solid/liquid ratio, amorphous silica obtained during calcination is dissolved, and little alumina goes into solution. This leaching step must be carried out on the crushed residue, and it is therefore advantageous to carry out the crushing upstream of the thermal pre-treatment step. The comminution process and target particle size may be similar to those used in the traditional bayer process. The pre-treated residue may also be reground prior to introduction into the bayer process.

The leachate may have a sodium hydroxide concentration of between 100g/L and 150g/L, preferably between 125g/L and 135 g/L. Below this minimum concentration, sufficient silica cannot be eliminated, while above this maximum concentration too much aluminium is dissolved, which reduces the yield of the process and/or impairs the economic efficiency of the process. The temperature is at least 85 ℃ and preferably at least 95 ℃. The temperature need not exceed 100 ℃. The duration of leaching may generally be between 30 minutes and 120 minutes. The calcined mineral charge may generally be between 10g/L and 200g/L, preferably between 15g/L and 150 g/L.

The process of pre-treating a physically enriched residue of natural bauxite by a process comprising calcination and leaching in sequence produces an intermediate product referred to herein as a "pre-treated residue", which is chemically and mineralogically distinct from the initial residue of the physical enrichment of bauxite. Accordingly, a first object of the present inventionIs a process for the preparation of alumina trihydrate or alumina from a physically enriched residue of bauxite, which has been pretreated by a process comprising calcination and leaching. Advantageously, the Al of the physically enriched residue₂O₃/SiO₂The mass ratio is greater than 1.5, preferably greater than 2.0, and even more preferably greater than 2.5. Advantageously, the alumina mass content of the physically enriched residue is greater than 35%, preferably greater than 40%, and even more preferably greater than 45%. The physically enriched residue has a silica content of less than 45% by mass, preferably less than 40% by mass, and even more preferably less than 35% by mass.

Said pretreatment of the bauxite process-enriched residue results in a so-called pretreated residue, advantageously having an Al greater than 3.0, preferably greater than 4.0, and even more preferably greater than 5.0₂O₃/SiO₂Mass ratio. The pretreated residue is characterized by a loss on ignition of less than 5 mass%, preferably less than 3.0%, and even more preferably less than 2.5%. Advantageously, the pre-treatment residue is also characterized by the absence of diaspore and the presence of amorphous silica.

The step of calcining the aluminum trihydrate is optional: if the process according to the invention is intended to obtain aluminium trihydrate as a commercial product, it is sufficient to dry the aluminium trihydrate obtained in step (d). Step (e) is necessary if the process is intended to obtain alumina.

Advantageously, the pre-treatment residue has an Al of more than 3.0, preferably more than 4.0, and even more preferably more than 5.0₂O₃/SiO₂Mass ratio. Advantageously, the alumina content of the pre-treatment residue is greater than 60% by mass, preferably greater than 65% by mass and even more preferably greater than 70% by mass. The pretreated residue has a silica content of less than 12% by mass, preferably less than 10% by mass, even more preferably less than 8% by mass.

In an advantageous embodiment of the process according to the invention, the liquid phase from step (d) is reintroduced into the aqueous sodium hydroxide solution used in step (a). Calcination causes a chemical and crystallographic transformation of the residue. More particularly, diaspore (which is a form of most (and usually almost all) of the alumina in the low a/S ratio bauxite) is converted for the most part (usually all) to alpha alumina. This transformation is accompanied by the exit of certain volatile substances present in the residue and/or formed during said chemical and crystallographic transformation. The loss on ignition is a readily determinable parameter that comprehensively expresses the state of this chemical and crystallographic transformation during calcination.

Under the usual conditions of the so-called bauxite digestion step of the bayer process, the calcined residue has a better dissolution rate of aluminium, in particular of aluminium in aluminosilicates, and a better dissolution rate of silicon under milder conditions than the digestion step of the bayer process. Thus, treating calcined bauxite by leaching with aqueous sodium hydroxide can dissolve silica under milder reaction conditions than the digestion step of the bayer process.

Another object of the present invention is a process for the manufacture of alumina trihydrate or alumina from bauxite physically enriched residue, the physically enriched residue being pretreated by calcination and leaching to obtain a pretreated residue, the process comprising the steps of:

(i) physically enriching bauxite to obtain enriched bauxite and the physically enriched bauxite residue,

(ii) the pretreatment is carried out on the bauxite physical enrichment residue, and the pretreatment sequentially comprises the following steps:

o a step of calcining the mixture,

o is leached with an aqueous solution of sodium hydroxide,

o separating solids from the aqueous leach phase, the separated solids being the pre-treatment residue,

(iii) the pre-treatment residue is treated by the method described above.

Al of the residue before pretreatment₂O₃/SiO₂The ratio is between 1 and 7, preferably between 1 and 5.5, even more preferably between 1 and 4, and most preferably between 1 and 3.

Another object is an alumina obtainable by one of the processes according to the invention.

Another object is a device for implementing the method according to the invention, comprising: a unit for pretreating bauxite mineral rich residues by calcination and leaching, enabling the residues to be converted into pretreated residues; and a unit for preparing alumina from the pretreated residue according to the process of the invention, characterized in that:

-the pre-processing unit comprises:

o at least one calciner for calcining the residue,

o at least one leaching unit for leaching the calcined residue with an aqueous solution of sodium hydroxide (known as leachate), and

o at least one solid-liquid separation unit for separating the calcined and leached residue from the leachate;

-the unit for manufacturing alumina from the pre-treatment residue comprises:

o at least one chamber for treating said pre-treatment residue with an aqueous solution of sodium hydroxide (called "Bayer liquor") at a temperature of at least 100 ℃,

o at least one solid-liquid separation unit for separating a solid residue from the bayer liquor;

at least one crystallization unit for crystallizing aluminum trihydrate from the bayer liquor by adding aluminum trihydrate seed crystals;

o at least one solid-liquid separation unit for separating the crystallized aluminum trihydrate from the bayer liquor;

o optionally at least one calcination unit for converting the aluminum trihydrate to alumina.

Another object of the invention is a device for implementing the method according to the invention, comprising:

-a unit for pretreating bauxite mineral matter beneficiation residue by calcination and leaching, so as to be able to convert said residue into pretreated residue; and

a unit for preparing alumina from the pre-treatment residue to carry out the process according to the invention,

it is characterized in that the preparation method is characterized in that,

-the pre-processing unit comprises:

o at least one calciner for calcining the residue,

o at least one leaching unit for leaching with an aqueous solution of sodium hydroxide (known as "leachate")

Leaching the calcined residue, and

o at least one solid-liquid separation unit for separating the calcined and leached bauxite from the leach solution;

-the unit for preparing alumina from the pre-treatment residue comprises:

o at least one chamber (for example an autoclave or a tubular device) for treating the pre-treatment residue with an aqueous sodium hydroxide solution (called "bayer liquor") at a temperature of at least 100 ℃, o at least one solid-liquid separation unit for separating solid residues (called "red mud") from the bayer liquor;

o at least one crystallization unit for crystallizing aluminum trihydrate from the bayer liquor by adding aluminum trihydrate seed crystals;

In this apparatus, the bayer liquor from the solid-liquid separation unit for separating crystalline aluminum trihydrate from the liquid phase is recycled to the digestion step.

Drawings

In fig. 1 and 2, the three digit notation refers to a physical object (equipment, component or product) and the four digit notation refers to a process step. After the phase separation step, the letter "L" represents the liquid phase and the letter "S" represents the solid phase.

Fig. 1 shows a simplified diagram of a bayer process according to the prior art.

Figure 2 shows a simplified diagram of an embodiment of the method according to the invention.

Detailed Description

1.Terms and background

The term "bauxite physical enrichment residue" refers to any residue resulting from the application of at least one physical enrichment process to bauxite.

By physical enrichment process of bauxite is meant any physical process aimed at increasing the alumina content in natural bauxite that can be extracted by the bayer process. These physical enrichment processes are processes that do not involve chemical conversion of bauxite and are therefore contrary to chemical enrichment processes of bauxite (e.g. processes of calcination or leaching of bauxite). For example, these physical enrichment methods do not include the calcination-leaching process of bauxite with a low alumina content ("roast-leaching") studied in the scientific literature, nor the various calcination processes in reducing media (processes which can be carried out with carbon monoxide, see CN103614547 or CN104163445, or processes which can be carried out with carbon, see CN101875129), nor the calcination processes of bauxite with a high content of Si and Fe for the conversion of iron into separable phases by a magnetization process (see k. yilmaz et al, "Pre-benefication of low-grade diaspore ore by reduction roasting of bauxite", published in 2015. int. j. chemical, Molecular, Nuclear, Materials and mechanical Engineering, volume 9, page 1023).

The process for the physical enrichment of bauxite includes in particular all mechanical processes. They are commonly used for pulverized bauxite. They may be based on particle size sorting, e.g. by sieving, or density sorting, or they may exploit differences between the wettability of the liquid medium for different types of particles, depending on their size and/or their morphology and/or their chemical composition.

Physical processes for the enrichment of bauxite are known per se and are used industrially. They do not form part of the present invention. The process of physical enrichment of bauxite is aimed at producing at least two fractions: the first fraction is referred to as "enriched bauxite" and the second fraction is referred to herein as "bauxite physical enrichment residue". To the extent that the physical enrichment process for bauxite is intended to increase the proportion of alumina that can be extracted by the bayer process, the enriched bauxite contains more recoverable alumina than can be obtained from natural bauxite, and the residue contains less alumina that can be extracted by the bayer process than the bauxite from which the residue is obtained. According to the prior art, this residue is no longer used for extracting metallurgy; it is either landfilled as final waste or recovered as backfill material.

According to the present invention, as described below, the physically enriched residue of bauxite needs to be pretreated in two steps in order to be able to be recovered in the bayer process. The term "pretreated residue" refers herein to a residue that has been pretreated by both calcination and leaching. The terms "calcined residue" and "calcined, leached residue" are used to denote intermediate products from each of these two steps.

In the introduction to the improved bayer process for digestion of pre-treated residues according to the invention, the designation "Na" is used₂O ctc "refers to the useful (" caustic ") portion of sodium hydroxide and is known by the name" Na₂O cbte "means Na₂O corresponds to the portion of carbonate residue ("cbte"); this distinction is possible by analyzing the liquid phase by means of pH-titration according to methods familiar to the person skilled in the art.

2.Standard bayer process

According to the followingFIG. 1 shows a schematic view of aThe bayer process of the prior art shown explains the invention in detail. Bauxite from a bauxite mine is crushed (step 1100) in the presence of a liquid phase, which is sodium aluminate, as explained in more detail below. The pulverization is carried out to increase the specific surface area of the bauxite which can utilize the liquid phase action during the action of digesting the bauxite. It is often the aim to reach particle sizes of several hundred μm. Comminution is carried out in the form of an emulsion or in the form of a solid with the addition of lime (step 1102). Lime has a triple effect: (i) lime reduces the consumption of sodium hydroxide during the digestion of bauxite because it favors the precipitation of soluble silicates in the form of calcium aluminosilicates rather than sodium aluminosilicates (otherwise some of the sodium is carried away from the sodium hydroxide, which is more expensive than lime); (ii) the lime promotes the dissolution of aluminum and improves the extraction efficiency of aluminum oxide in the dissolution process; and (iii) lime improves the decantation of the slurry after the action, since it promotes difficultiesTo decant and filter the goethite to a more crystalline hematite.

The crushed bauxite is then acted upon with an aqueous solution of sodium hydroxide (step 1110) in an autoclave or tubular exchanger under pressure and at elevated temperature. This action (referred to as "digestion") results in the digestion of a portion of the bauxite (step 1120), more precisely, the soluble portion of the aluminum mineral that forms aluminate ions (particularly alumina, as either a monohydrate or trihydrate). In many cases, the digestion is carried out at a temperature between 250 ℃ and 270 ℃ in a closed autoclave or tubular exchanger. The aqueous sodium hydroxide solution is actually an aqueous sodium aluminate solution. Na is usually used at a concentration of 235g₂O/L and 245g Na₂Sodium hydroxide between O/L.

The temperature range of 250 c to 270 c ensures that all the soluble alumina contained in the bauxite (including the diaspore fraction, which is the least soluble fraction of the aluminum oxide and which may vary widely) is dissolved. In particular, karst-type bauxite, which is the main application field of the present invention, requires this temperature range. Some plants using bauxite of the karst type are even designed to operate at temperatures up to 280 ℃ in order to be able to adapt, if necessary, to the use of bauxite with very specific composition.

During this digestion step 1120, the bauxite may be contacted with a preheated liquor (a so-called dual stream process) or a suspension of bauxite in bayer liquor is formed prior to heating (a so-called single stream process). In some plants, the digestion step 1120 is carried out in two steps, each of which is operated at a different temperature, in order to dissolve the easily soluble fraction first, and then the solid residue of the first step at a higher temperature. This double stripping variant may reduce the use of energy, but requires greater investment and complicates the process.

As the suspension expands (step 1124), a portion of the water evaporates (self-evaporates) through the successive expansion steps.

During the run-out at ambient pressure (step 1130), the residue (referred to as "red mud") separates from the liquid phase (liquid); flocculants are added to increase the rate of phase separation and improve the clarity of the liquor (i.e. the residual amount of dry matter in the liquor). Red mud contains all crystalline phases from bauxite that are not reactive to digestion (1120) and are formed in the bayer cycle. Recovering the red mud (step 1140) and washing with water (step 1142) to recover a maximum amount of liquid; this washing is usually carried out in countercurrent with the raw water (to minimize the amount of water used); this is followed by a decantation and/or filtration step (not shown in the figure). Red mud is a powdery residue, is not easy to recover, and is often stored in a specific storage facility.

The liquid phase ("L") from phase separation step 1130 is a sodium aluminate solution. After dilution (step 1150), the aluminum trihydrate is crystallized (step 1170) by cooling the aluminate and adding seed crystals (also referred to as "primers") of the aluminum trihydrate (step 1160). This crystallization step is referred to in the industry as "decomposition"; it takes about 40 hours. The wash water from the red mud is reused for dilution in cold water (step 1150).

This step 1170 requires a certain degree of expertise familiar to the person skilled in the art to adapt to the numerous parameters of the process (saturation of the liquid at the decomposition inlet, Na)₂Concentration of O and different kinds of impurities, temperature at the beginning and end of decomposition, surface area of the primer, crystallization technique, particle size classification) and suitable properties of the desired alumina product; the physicochemical phenomena involved are mainly related to nucleation (spontaneous formation of fine particles in suspension), agglomeration of fine particles and size classification by cyclone and/or decantation.

The precipitated trihydrate is isolated by decantation and filtration (step 1180) using various known techniques; the trihydrate is recovered (step 1190). Most of the trihydrate is recycled in the decomposition step 1160, and the remaining portion is dried (step 1192) and calcined (step 1194) to alumina. The alumina is stored (step 1196) for delivery to the customer site. The drying step (step 1192) is typically performed as the first step of calcination (step 1194) performed in several stages. During the temperature increase, starting from about 100 ℃, the steep water is first removed, followed by the water for the formation of the trihydrate (about 1000 ℃); the temperature is then increased further to obtain the desired crystal structure. Two techniques are mainly adopted: circulating Fluidized Bed (CFB) in-kiln calcination, hot Gas Suspension Calcination (GSC). The old plant still has a rotary kiln, which is also suitable for performing this step.

The liquid phase from phase separation step 1180 is an aqueous sodium hydroxide solution, more dilute than the aqueous sodium hydroxide solution used in step 1110, because different inlets of water (red mud wash water (step 1142) and trihydrate, dilution water (step 1150)) flow into the stream. For this reason, it must be concentrated by evaporation of the water (step 1210) in order to be recycled (step 1220) in the sodium hydroxide solution used in the dissolution step (step 1120). It is also reused (step 1222) in the wet grinding step of bauxite (step 1100).

The trihydrate obtained in step 1190 may be washed before drying; this wash water can be used again for red mud washing in step 1142 (not shown in the figure).

In the bayer process according to the prior art, sodium hydroxide is consumed in the processing of bauxite to produce alumina. More precisely, this consumption is related to three mechanisms: (i) forming an insoluble sodium aluminosilicate phase during the action (digestion step 1120); (ii) residual sodium hydroxide is carried over by the slurry despite the washing (step 1142) (1140); (iii) coprecipitated with alumina in the crystallization stage (1170). These losses must be compensated for by the addition of fresh sodium hydroxide (step 1110). As much as possible of the washing liquor containing sodium hydroxide, including in the chemical cleaning of tanks and pipelines, is recycled in the bayer liquor.

3.The invention relates to pretreatment of bauxite physical enrichment residue.

3.1SUMMARY

In thatFIG. 2An embodiment of the method according to the invention is illustrated. It involves the pretreatment of the physically enriched residue of bauxite. The pre-treated residue is introduced into the bayer process. The bayer process steps, labeled 11xx and 12xx in fig. 1, are denoted by the symbols 21xx and 22xx in fig. 2, while the symbols of the pre-treatment step are 20 xx.

According to a very advantageous embodiment of the invention, certain operating conditions of the bayer process are adapted to the chemical and mineralogical composition of the pre-treatment residue; this will be explained in more detail below. The pretreatment residue is a product which does not occur in nature, and it is necessarily a product produced by an industrial process, i.e., a pretreatment process. Its chemical composition is different from that of the residue from which it was physically enriched, and differs in two fundamental characteristics: it has a higher a/S ratio (because it contains less silicate) and it contains only little water of crystallization. Furthermore, its mineralogical composition is different as it undergoes a transition in different pre-treatment steps, as will be explained in more detail below.

The main difference relates to diaspore and kaolinite. During calcination, diaspore is dehydrated and converted primarily to alpha alumina, which is soluble in sodium hydroxide under the digestion conditions of the bayer process. Also, during calcination, the kaolinite undergoes dehydration and conversion to metakaolinite (as described in section 3.3 below), which allows for dissolution of the silica by sodium hydroxide during leaching.

Low crystal water content is another factor in distinguishing the chemical composition of the pretreated residue from the chemical composition of the physically enriched residue of bauxite. The loss of crystal water is a major parameter of the loss on ignition (mass). For example, the loss on ignition of physically enriched residue of bauxite is typically greater than about 10%, typically between 10% and 20%, while the loss on ignition of pretreated residue according to the present invention is less than 4.0%, preferably less than 3.0%, and even more preferably less than 2.0%. Loss on ignition is a parameter known to those skilled in the art; further explanation is given in section 3.3 below.

According to the invention, bauxite from bauxite mines is crushed (step 2000) after the addition of water (step 2002) and then subjected to at least one physical enrichment process (step 2003). Two solid intermediate products are produced from the process, namely separated enriched bauxite (step 2005) and a physically enriched residue of bauxite. The physical enrichment process is typically a wet process, in which case it may be necessary to dry one and/or the other of the intermediates; in any case, this is necessary for the physically enriched residue of bauxite. In a known manner, enriched bauxite is used as raw material in the bayer process, as described in section 2 of the description above.

The physical enrichment process (step 2003) may be a wet screening process (typically performed on a vibrating screen); in this process, the fine particles highly concentrated with silica are carried away by the washing water to be separated from the particles rich in alumina. Flotation may also be used; in this process, finely pulverized bauxite (less than 200 μm) is floated using chemical additives to adsorb target minerals in the float, thereby enabling their separation from heavier particles. It is also possible to use gravimetric methods, using one or more liquids of suitable density to separate the supernatant phase from a suspension containing particles of different densities. All of these physical enrichment processes used in an industrial manner produce a residue of fairly similar composition for a given bauxite (this residue is known as "tailings").

Regardless of the nature of the physical enrichment process that produces the residue, they are advantageously in comminuted form, preferably having an average particle size of less than 200 μm, and even more preferably 100% of the particles pass through a sieve having openings of 200 μm.

According to an essential feature of the process of the invention, the physically enriched residue of bauxite (step 2006) is then subjected to a pretreatment. The pretreatment is a chemical pretreatment because it causes chemical conversion of the residue. It comprises two steps.

The method of pre-treating the residue comprises a first step, namely calcination (step 2010). This produces an intermediate product that is preferably referred to herein as a "calcined residue". The second step of the pretreatment is leaching: an aqueous sodium hydroxide solution (step 2020) is added to the calcined residue and leaching is performed (step 2030). This step is usually carried out in suspension at elevated temperature.

After phase separation (step 2040), the solid phase, referred to herein as "calcined leach residue" or more simply "pre-treatment residue", is recovered and introduced into the bayer process; depending on the particle size obtained during comminution in step 2000, it may be necessary to comminute the residue again (step 2100, not shown in fig. 2). The liquid phase from the phase separation in step 2040 is treated with lime to precipitate silicates (step 2050). After further phase separation (step 2060), the residue, i.e., white mud, is recovered (step 2070). The liquid phase from the phase separation of step 2060 is aqueous sodium hydroxide; it is recovered (step 2080) and partially recycled to the working solution of the bayer process. Advantageously, the lime (2052) is introduced in the form of milk of lime.

This method of pretreatment of physically enriched residue of bauxite ore may include a variety of variations in accordance with the present invention. For example, phase separation step 2060 may be followed by an additional filtration step of the liquid phase (a step not shown in FIG. 2, denoted here as 2062). It may comprise washing the lime mud after the phase separation step 2060 (a step not shown in fig. 2, here denoted by 2064), the wash water being recycled to step 1150. These two variants can be combined.

As explained in detail below, the treatment of the pretreatment residue in the bayer process may be carried out in the same equipment, i.e. using the same equipment, and according to the same process scheme as the treatment of the enriched bauxite. (the only exception is the evaporation step 2210, which may be omitted in some variants of the method according to the invention). However, if the same operating parameters (e.g., duration, temperature, and/or concentration of sodium hydroxide) are used in the process, the results obtained are different than those obtained using enriched or non-enriched bauxite. To this end, in some very advantageous embodiments of the invention, the operating parameters are modified with respect to the usual functioning of the bayer process for some steps of the bayer process carried out with the pretreated residue according to the invention.

The inventors have found that the calcination temperature of the physically enriched residue of bauxite (step 2010) strongly influences the yield of alumina extracted from the pretreated residue. According to the invention, the calcination temperature must be greater than 980 ℃. For calcination temperatures of 980 ℃ or less, the kaolinite is activated and not fully converted, which reacts during the leaching step (step 2030) to produce insoluble zeolite-like compounds. For this reason, calcination temperatures greater than 990 ℃ are preferred. According to an advantageous embodiment, the calcination temperature is greater than 1000 ℃. For temperatures between 1010 ℃ and 1035 ℃, the transition of the kaolinite is complete; temperatures between 1020 ℃ and 1030 ℃ are preferred. The duration of this calcination in this temperature range between 980 ℃ and 1050 ℃ may be between 15 minutes and 60 minutes, preferably between 20 minutes and 45 minutes. In an advantageous embodiment, the calcination is carried out at a temperature between 1015 ℃ and 1045 ℃ for a period of time between 20 minutes and 45 minutes. For example, calcination is carried out at 1030 ℃ for 30 minutes. The calcination step typically includes a temperature ramp and a cooling ramp; the time shown is the residence time within the specified temperature range.

The calcination process is followed by leaching, as will be explained below.

The modified pre-treated residue resulting from this calcination-leaching process (roast-leaching) can be introduced directly into the bayer process. According to an advantageous embodiment of the invention, the bayer process is modified. More precisely, certain operating parameters are modified, which makes it possible to significantly reduce power consumption, etc.

3.2Detailed description of the preferred embodiments

For purposes of illustrating embodiments of the present invention, certain steps of the pretreatment process are described in detail herein.

Calcination of(step 2010)

The calcination of the physically enriched residue of bauxite in step 2010 may be performed in a rotary kiln or a static kiln. At the above temperature, gradual heating makes it possible to remove the impregnation water from the residue, then to remove the constituent water from the crystalline phases in the residue, and then to transform these phases. Under these conditions, it was observed that:

-the silica present in silicate form is largely converted into amorphous silica;

-conversion of diaspore and boehmite phases to "alpha" alumina;

iron in the form of goethite (FeO (OH)) is converted to hematite (Fe) after calcination₂O₃)；

The phase containing carbon, carbonate and sulphur is mainly CO after thermal decomposition₂And SO₂Is present in the volatile fraction.

Leaching out(step 2030)

After calcination, the calcined residue is immersed in a sodium hydroxide solution (step 2020). This leaching step (step 2030) is capable of dissolving the converted silica as well as certain impurities. The sodium hydroxide content of the liquid phase may be between about 70g NaOH/L and about 160g NaOH/L, preferably between about 90g NaOH/L and about 150g NaOH/L, and even more preferably between about 110g NaOH/L and about 140g NaOH/L. For example, a content of 129g NaOH/L was successfully used. This solution can be obtained from a mixture of recovered sodium hydroxide and 50% sodium hydroxide lye, the amount of which is adjusted to obtain the desired concentration for leaching. Below 70g/L the leachable silica is dissolved too little, too much residence time (i.e. contact time between solid and liquid phase) is required and the stock sodium hydroxide solution circulating in the process unit is diluted too much. Above 150g/L, the risk of alumina dissolution leading to aluminium loss becomes significant. The temperature of the aqueous sodium hydroxide solution is generally between 80 ℃ and 120 ℃; if the temperature is too low, the silica does not dissolve well, and if the temperature is too high, the alumina dissolves easily.

For example, the calcined residue and sodium hydroxide solution (2010) may be introduced into a stirred reaction tank to obtain a catalyst containing about 80kg/m³Initial suspension of solids. Reaction temperatures of about 100 ℃ are suitable; the residence time at the reaction temperature may be about 45 minutes.

Phase separation(step 2040)

The phase separation in step 2040 may be performed by filtration in a filter, e.g. a filter press, which is supplied with the suspension from the leaching reactor (2030). The solid residue is "calcined, leached residue" or "pretreated residue"; about 10% by mass of the leachate is acceptable as the residual impregnation. The liquid phase is a liquid containing dissolved silica resulting from the leaching process (2030); it is purified by the addition of lime (2052). Advantageously, the raw material is quicklime (CaO) or milk of lime (Ca (OH)₂). Preferably, fine-grained quicklime is used, the CaO content being at least 85%; typically it contains between 85% and 95% CaO. This lime (about 100kg CaO/m) can be slaked by hot water in a stirred tank reactor³) To produce lime milk.

Precipitation of silicates(step 2050) and phase flow

The silicate preparation step (step 2050) may form insoluble calcium silicate. The precipitation of silica can be carried out in a reaction tank in the presence of quicklime or milk of lime (100g CaO/L) with stirring at 100 ℃ for 2 hours. Advantageously, CaO SiO₂Is between 1.1 and 1.5. At the end of the run, the suspension obtained generally contains about 35kg/m³To 50kg/m³Preferably 39kg/m³And 46kg/m³The solid in between.

Advantageously, the phase separation (step 2060) may be carried out by decanting the suspension. According to the invention, the clarified liquid phase (overflow) is advantageously recycled to the sodium hydroxide circuit of the process, preferably partially to leaching (2030), and partially to upstream of the digestion step (2120) of the bayer process.

Calcium silicate (typically 600kg solids/m) is extracted from the decanter³To 700kg solids/m³) A thickened suspension of constituents (underflow), called white mud (2070). It may be supplied to a filter, such as a belt filter, which is washed in water in a orderly fashion (step 2072) to reduce the concentration of the impregnation fluid. The washed lime mud may be impregnated with about 10% of the diluent; the sodium hydroxide concentration of such impregnation solutions is generally about 6g NaOH/L. to 10g NaOH/L. The white mud consists of calcium silicate hydrate, the composition of which is close to that of tobermorite (Ca. Ca)_4.31Si_5.51Al_0.5O₁₆(OH)₂x4H₂O). It may be directed to an intermediate storage while waiting for recovery.

The wash water of the silicate sludge (lime mud 2070) recovered from the end of step 2072 may be added again to the circuit of the liquid phase 2080 obtained in step 2060 for the leaching step (step 2030) and the bayer process (step 2120). In the latter case, it is necessary to adjust again its sodium hydroxide content (step 2110), which will become lower than the initial content in step 2030 (e.g. 129g NaOH/l). This readjustment was carried out by adding an aliquot of 50% sodium hydroxide lye.

The pretreatment according to the invention results in a certain loss of sodium hydroxide due to occlusion of sodium in the precipitated silicate and impregnation of the lime mud. This loss must be compensated for by adding sodium hydroxide lye, typically 50% sodium hydroxide lye (step 2110). However, as described below, the improved bayer process according to the present invention consumes less sodium hydroxide per ton of alumina produced than the conventional bayer process.

The bayer liquor is recycled (step 2200); it consists of a mixture of liquids from the trihydrate filtration step (step 2180) which may or may not be concentrated by evaporation of water (step 2210) and addition of sodium hydroxide lye (step 2110).

3.3Significance of ignition loss

As is well known, loss on ignition is a parameter that is an essential part of the normal characterization of bauxite; this value, expressed in mass percentage, appears on the analytical certificate attached to each delivery of bauxite for the bayer process. The loss on ignition is generally determined by calcination at 1060 ℃ for 2 hours after preliminary drying at 105 ℃. Even if it is considered alone that the possible presence of oxidation reactions leads to an increase in mass, the calcination of bauxite always leads to a net loss of mass, caused by the exit of volatile substances. This loss of volatile substances is the result of physical phenomena (in particular sublimation) and chemical phenomena (in particular thermal decomposition, such as dehydration, dehydroxylation, thermal dissociation and reduction). More precisely, the loss on ignition corresponds mainly to the elimination of the constituent water (i.e. water molecules bound into the crystal structure), carbon dioxide from organic and mineral carbonates and certain other volatile compounds, in particular sulphur oxides.

The same reasoning applies to the physically enriched residue of bauxite. The loss on ignition of this residue depends on its chemical and mineralogical composition. The ignition loss value is generally less than 20 mass%, preferably less than 18%, and even more preferably less than 16%. Under the conditions shown, the loss on ignition can be determined by simple weighing before and after calcination. Differential thermogravimetry may also be used, which may also characterize the mineral species present in the residue.

In particular, it is observed that the loss on ignition of the residue after calcination is generally less than 0.50%, and generally less than 0.40%, but it increases slightly after leaching due to the formation of hydrated phases.

4.The improved Bayer process according to the invention uses a pre-treatment residue

The phase separation (step 2040) makes it possible, preferably after filtration, to separate the solids from the liquor, so that the pretreated residue can be used in the digestion step of the bayer process (step 2120). The inventors have made numerous observations that have led them to modify certain steps of the bayer process; this modification is an essential feature of the present invention.

Dissolution of(step 2120)

Digestion involves dissolving the alumina phase contained in the pretreated residue in a sodium hydroxide liquor. The alpha alumina contained in the pre-treatment residue produced during calcination, as well as the pre-existing soluble alumina, is dissolved by the bayer liquor at high temperatures. This step may be carried out under temperature and pressure conditions similar to those of the conventional bayer process, namely: in closed autoclaves or pressurized tubular systems (about 50 to 60 bar), the temperature is generally between 250 ℃ and 270 ℃. Advantageously, the heating is carried out by gradually increasing the temperature up to the reaction temperature. Advantageously, the residence time at the reaction temperature is between 30 minutes and 60 minutes, preferably between 30 minutes and 50 minutes, and even more preferably between 35 minutes and 45 minutes.

According to an essential feature of the process of the invention, it is possible to use a sodium hydroxide concentration for leaching the pretreatment residue that is significantly lower than the sodium hydroxide concentration used for ordinary bauxite in the conventional bayer process. More specifically, the concentration is 140g Na₂O/L and 200g Na₂Between O/L, preferably 155g Na₂O/L and 190g Na₂Between O/L, and even more preferably 160g Na₂O/L and 180g Na₂Between O/L. Advantageously, the concentration is continuously monitored by measuring the conductivity of the liquid; it can also be chemically analyzed in the laboratory.

In one embodiment, the inventors have found that the circulation flow rate of the working liquid is despite the lower concentration of caustic soda (162g/L versus 240g/L) of the digestion liquidHigher (12.11 m)³T is relative to 8.52m³T), but the alumina extraction is still high, by adjusting the plant operating parameters, such as the saturation of the liquid (alumina concentration and R)_P(defined below)), lime addition (8% to 10.4%) and temperature (260 ℃), the silica-deficient alumina extraction being higher than 96%.

Evaporation of(step 2210)

At the outlet of the digestion, the suspension is expanded, i.e. brought to atmospheric pressure by continuous expansion; this operation allows evaporation of large amounts of water (self-evaporation).

Since the process according to the invention uses a bayer liquor with a much lower concentration of sodium hydroxide than in the conventional bayer process, the amount of water to be evaporated is much lower. In some cases, the evaporation during autoclave expansion (step 2124) at digestion (2120) is sufficient to maintain the concentration of recycled aluminate liquor (2220); the evaporation step 2210 may then be omitted. The evaporation step 2210 consumes heat energy and requires a large investment in evaporators; the fact that this step can be minimized or even omitted has significant economic advantages.

It should be noted that although there are plants that employ the conventional bayer process without performing the evaporation step 1210, i.e., plants that use only a particularly small amount of red mud produced from bauxite, it is not at all possible to omit this step in the case of digestion of low a/S ratio bauxite using the conventional bayer process.

Decantation of red mud(step 2130)

At the end of the digestion, the suspension is diluted with aluminate liquor from the first stage of washing the red mud in step 2140. The dilution is adjusted according to the input of the washing water. This makes it possible to obtain a liquid concentration suitable for solid-liquid separation and crystallization in step 2160.

Decantation is carried out in an apparatus called "decanter" which has a tank of large diameter (most having a flat or conical bottom) and provides slow agitation to allow separation. Decantation is optimized by using additives known as flocculants to increase the settling rate of the solid particles.

In the process according to the invention, there are no certain phases, such as goethite, which is transformed during the calcination of the bauxite in step 2010 and is known to interfere with flocculation, which makes it possible to reduce the amount of flocculant used.

Typically, the thickened suspension (underflow) is sent to the first washing stage. The clarified liquid (overflow) is sent to a so-called "safety" filtration, aimed at removing very fine mud particles, to ensure that the liquid is free of impurities to facilitate crystallization.

Washing red mud(step 2140)

The washing of the red mud in step 2140 is preferably carried out in countercurrent; the wash water is introduced into the final stage of the scrubber line. The scrubber line can be completed by filtering the mud from the final scrubber using a filter press. The use of a flocculant may improve settling to ensure better washing of the mud.

The inventors have found that, for the pre-treatment of the residue by the process of the invention, the appropriate adjustment of the main parameters of the bayer process, in particular of the effect (residence time, liquor saturation, amount of lime added, etc.), enables the use of an effect liquor of low concentration of caustic soda (about 160 to 170g/L for the conventional effect of 238 to 240g/L) to maintain an excellent alumina dissolution rate (greater than 96% alumina, minus silica). These results, together with the improved efficiency of the mud washing, make it possible to operate the bayer liquor cycle with low caustic concentrations, with a significant saving in energy and maintenance costs of the evaporation plant. By omitting the evaporation step 2210, the investment in evaporation equipment can be reduced in case of a new production line.

Desiliconizing and white mud treating the leach liquor

The desilication step (2050) by silicate precipitation (also called "desilication") produces a specific slurry ("lime mud") that does not occur in the traditional bayer process. The slurry is decanted (step 2060). After separation (step 2070), water washing (step 2072) and drying, it may be landfilled as final waste or possibly recycled for other products.

Overview of the final waste products produced by the method according to the invention

The modified bayer process according to the invention uses physically enriched residues of bauxite as raw material, pretreated by calcination and leaching according to the invention, producing red mud in an amount comparable to that of high-grade bauxite. Thus, the overall result of the final residue amount is significantly positive from the initial physical enrichment treatment of bauxite to the obtaining of alumina from the pretreated residue by the bayer process (by integrating the amount of white mud produced during desilication of the leachate). In other words: the amount of final residue produced by recovering the pre-treatment residue (i.e. red mud and white mud) is much smaller than the amount of bauxite physical enrichment residue involved in the process according to the invention, from which it is known that the process according to the invention has the advantage that it proposes a process for recovering the residue that was previously the final residue (i.e. the physically enriched residue of bauxite) under economically viable conditions.

5.Advantages of the method according to the invention

As mentioned above, the method according to the invention has a number of advantages. Its main advantage is that it allows the use of bauxite enrichment residue in the bayer process. These residues cannot be recovered by known industrial techniques in the production of alumina but must be landfilled. This improves the overall extraction results of bauxite, especially bauxite with low a/S ratios.

Thus, it is possible to increase the recoverable reserves of bauxite in economic terms (micro-or macro-economics) by allowing the use of mineral resources that could not be used by the processes according to the prior art under competitive economic conditions. In fact, the process according to the invention makes it possible to extract alumina from bauxite physically enriched residues which cannot be recovered in this way by the processes of the prior art. This increases the value of the bauxite deposit and reduces the amount of residue that must be disposed of at the extraction site.

It is of course possible and also within the scope of the invention to supply bayer plants that have been modified according to the method of the invention with pre-treated residues that have not been pre-treated in the same place: the pre-treatment residue may come from a separate pre-treatment plant (e.g., located near the bauxite mine to reduce the bauxite transportation costs) or from an integrated plant (pre-treatment unit + bayer unit) where the pre-treatment residue is over-produced. However, this first embodiment with a separate pretreatment unit is not preferred, as in this case the liquid phase containing sodium hydroxide from the leaching step 2040 and the lime mud wash 2072 cannot be recovered by recycling in the bayer liquor.

Another advantage is that the pretreatment according to the invention removes not only silicon but also almost all the organic carbon and most of the sulphur naturally contained in the residue. Organic carbon is known to accumulate in sodium aluminate solutions, some of which may precipitate as oxalate on trihydrate. It is known that in the ideal case of a lower oxalate concentration in the bayer liquor, there is more room for adjusting the parameters of decomposition step 2160 (in particular the temperature, residence time and recirculation rate of the primers) in order to meet the requirements of obtaining a product with controlled particle size, particle size distribution and microcrystalline form. Furthermore, the yield of this crystallization step is higher if the oxalate concentration of the bayer liquor is lower. It can thus be seen that the introduction of a portion of the bayer liquor from the action of the pre-treated residue according to the invention into the circuit of the bayer liquor from the action of bauxite (even enriched bauxite) does not result in an increase in organic carbon in the bayer liquor. Also, the process according to the invention does not lead to an increase in the residual iron and silica content of the alumina.

Another advantage already mentioned above is that the amount of water to be evaporated in the water evaporation step (step 2210), which may in many cases be omitted, is reduced. This contributes significantly to energy saving.

The process according to the invention comprises an additional calcination step (step 2010) consuming thermal energy and sodium hydroxide. However, the sodium hydroxide can be recovered in large quantities in the bayer process, and the energy consumption of the calcination step is almost offset by the energy savings in the bayer process.

The process according to the invention can be used in an advantageous manner for bauxite physically enriched residues with an a/S ratio of between 1.3 and 12, preferably between 1.5 and 9. Advantageously, the A/S ratio of the non-pretreated residue is greater than 1.5. The lower limit is somewhat dependent on technical and economic considerations; this method can treat kaolinite (a/S ═ 1), but this has no economic advantage.

In economic terms, this method produces positive results. Furthermore, the significant reduction in the number of final residues, which tend to have negative economic value, also reduces their reprocessing and storage costs. As regards white mud (mainly silicates), they contain less heavy metals and other potentially toxic substances (if they go into solution) than red mud; their economic value is not necessarily negative. In fact, the mineralogy of calcium silicate in the form of tobermorite has shown its application, in particular in the field of construction.

Examples

1)Selection and analysis of residues

Four batches of bauxite (labeled A, B, C and D) were subjected to one or the other of two physical enrichment processes; these physical enrichment processes applied to these batches of bauxite are industrial processes known to those skilled in the art.

According to the first physical enrichment method, bauxite ore is crushed and then wet-screened on a vibrating screen. The finest particles with very high silica concentration are carried away and eliminated by the wash water; the remaining formed residue (residues A, B and C) will then be treated by the method according to the invention.

According to a second physical enrichment method, finely ground bauxite (average particle size of about 200 μm) is subjected to flotation using chemical assistants in order to adsorb the target minerals then found in the float, thus separating from the residue (residue D) mainly comprising silicate minerals. The float contains organic residues from the flotation aid that are adsorbed on the solid phase.

The chemical composition of these residues, as well as their crystal structure, was analyzed by X-ray fluorescence. The results are shown in tables 1 and 2.

Chemical analysis showed that the alumina concentration varied widely above 39% while the silica content varied between 12% and 31%. The a/S ratio varies between 1.3 and 4.4. X-ray crystallographic analysis shows that silicon is present in the form of layered silicates, such as kaolinite, muscovite and oolitic chlorite, and the remainder of the silica is present in the form of quartz. Aluminum is present in the form of diaspore, boehmite, gibbsite and phases bound to silica.

Residue a contained a large amount of aluminium hydroxide (49%) and a low proportion of phyllosilicates (14%), whereas residue B contained less aluminium hydroxide (30%) and a large amount of phyllosilicates (42%), the majority of which were formed from muscovite mica (20%).

The quartz content of the residues A and B was very high (17%; 14%). The quartz is converted to beta quartz during calcination and remains insoluble during leaching. On the one hand, the high quartz content and, on the other hand, the type of siliceous phase (muscovite) make these residues a and B unsuitable for the process according to the invention, since the recoverable fraction of aluminum is too small to be economically profitable.

Residue C is rich in aluminium hydroxide (54%); it contains little layered silicate (12.5%) and an average content of quartz (7%).

Residue D is more rich in phyllosilicates (64%), most of which are present in the form of kaolinite (55%); there is a small amount (26%) of aluminum hydroxide and a small amount (2%) of quartz. These residues C and D are chosen to be treated by the process according to the invention, since they contain a large amount of siliceous phase which is likely to be converted into soluble silica by thermal treatment.

2)Pretreatment of the residue

2.1First pretreatment of the residue: calcination of

The residues C and D were crushed (average particle size 100% less than 200 μm) and then calcined in a preheated muffle kiln at a temperature of 1030 ℃ for 30 minutes. At the end of the calcination, the residue is cooled in a dryer. Table 3 shows the composition of samples C and D after calcination. Their chemical composition was analyzed by X-ray fluorescence, their structure by X-ray diffraction, and their volatile content (loss on ignition — LOI) was analyzed by weighing before and after heating to 1060 ℃.

Crystallographic analysis of residue C after calcination showed that the aluminium-containing phase and the layered silicate disappeared, while quartz, hematite, rutile and anatase remained (and a small amount of harmunite (CaFe)₂O₄)). A very pronounced corundum phase was observed.

Crystallographic analysis of the residue D after calcination showed that the region between 2 θ ° and 28 ° corresponds to the less crystalline phase: kaolinite has been partially converted to amorphous silica and gamma-transition alumina. The appearance of the silica-alumina phase identified as mullite was observed. It is unusual for kaolinite (so-called kaolinite 1M) to be converted to mullite at 1030 ℃, it being conceivable for an intermediate compound to form, which decomposes to mullite at low temperatures; it may be dickite (also known as kaolinite 2M, approximate formula Al)₂Si₂O₅(OH)₄) And is converted to mullite at a temperature between 990 ℃ and 1020 ℃. The proximity of the diffraction peaks makes it difficult to distinguish the two kaolinites. The corundum phase resulting from the hydroxide conversion of aluminum is also observed to appear. In addition, muscovite, quartz, anatase, rutile and hematite phases were also detected.

2.2Second step of residue pretreatment: leaching out

Leaching of the calcined residue the suspension was subjected to a temperature of 100 ℃ for 60 minutes using pure sodium hydroxide solution (NaOH 130g/L) and a volume of 500 mL. The residue amount of the calcined residue C was 90g/L, and the residue amount of the calcined residue D was 30 g/L. The suspension thus obtained was kept at 100 ℃ for 1 hour and then filtered through a Millipore membrane filter (5 μm). The filtrate was stored in an oven at 90 ℃ and an aliquot was removed for analysis. The residue is calcined and leached residue; it is washed and dried.

Table 4 shows the chemical analysis of the calcined, leached residue obtained by X-ray fluorescence.

It can be seen that the residue resulting from leaching is rich in aluminium, since the A/S ratio of residue C changes from 3.9 to 6.1, while the A/S ratio of residue D changes from 1.5 to 3.9.

Mass loss confirms that leaching has removed most of the amorphous silica. The loss on ignition values after leaching were higher than the calcined residue, indicating the formation of a hydrated phase. Recombination of the dissolved phase into zeolite-type sodium aluminosilicate (Na) was observed₂O x Al₂O₃ x 2SiO₂ x 3H₂O). Na of residue D₂The higher content of O indicates that the composition contains sodium aluminosilicate.

Table 5 shows the chemical analysis of the leachate, obtained from the elements determined by ICP or chromatography. The leaching solution contains silicon dioxide; a small portion of the alumina is also dissolved.

The net results of leaching are given in table 6. It is noted that the silica dissolution rate of residue C is close to the maximum value achievable (43%): there was almost no quartz remaining after leaching, and there was almost no introduction of sodium hydroxide and almost no dissolution of alumina.

The silica solubility of residue D indicates that 75% of the silica in the layered silicate has been removed. The dissolution rate of alumina was high (19.3%).

The X-ray diffraction pattern shows that the phases still present in the residue C from calcination and leaching are quartz, hematite, rutile, anatase, muscovite and corundum. The same phase was observed in residue D, but a new phase, such as mullite and kaolinite 1A, was also observed.

Since the initial residue D has a very high content of kaolinite (55%), the amount of leaching is reduced compared to the residue C, since in this case incomplete conversion of the kaolinite is found under the calcination conditions applied. Sodium hydroxide (1.9% Na)₂O) showed the formation of sodium aluminosilicate (about 8%). It is speculated that the calcination conditions do not allow for metakaolin conversion, which is recombined with sodium hydroxide to form the zeolite. Tests carried out with the higher load of calcined residue D (90g/L) confirmed the presence of a silico-aluminous phase, which combines with sodium hydroxide to form sodium silicoaluminate (45%), resulting in a poor performance of the residue pre-treatment process: the desilication rate then dropped to 33%.

2.3Desiliconizing the leach liquor

The leachate containing dissolved silica was treated with an excess of milk of lime (150%) at 100g/L CaO and solid CaO. The lime milk was prepared by stirring at 70 ℃ for 90 minutes and then at 85 ℃ for 60 minutes. The desilication was carried out by stirring at a temperature of 100 ℃ for 2 hours. The silica precipitates as calcium silicate. The desilication yield can be between 80% and 90%.

The obtained silicate consists mainly of tobermorite (68%) and calcium products (calcite, portlandite). Tobermorite has the approximate formula Ca₅Si₆O₁₆(OH)₂x 4H₂O。

After washing and drying, these silicates can be reused, for example as base materials for the production of materials for the building and construction industry, in particular cement, insulation boards, etc.

3.Use of pretreated residues in bayer process

3.1Dissolution test of pretreated residues

The pre-treatment residues were tested to determine the solubility of alumina by the bayer process and to deduce the ore, sodium hydroxide and lime specific consumption achievable with these residues.

The pre-treatment residue (i.e. the residue calcined and leached) was acted on under pressure in a 150mL autoclave at 260 ℃ by concentrated sodium aluminate (known to the skilled person as a leachate) having the following composition:

Na₂O ctc 169.8g/L；Al₂O₃ 94.07g/L；SiO₂ 1.1g/L；Na₂O cbte 12.8g/L R_p＝Al₂O₃/Na₂O ctc＝0.554。

lime is in the form of lime milk (to obtain lime milk, quicklime is mixed with low R_pTogether with concentrated aluminate(s) to ensure the dissolution of the catalytic corundum and to reduce the amount of sodium hydroxide introduced in the final residue. The residence time at the action temperature was set to 40 minutes. Considering that the dissolution rate of extractable alumina is 100%, for the target saturation R at the end of the action_p＝1.10(R_p＝Al₂O₃/Na₂O ctc) to define the amount of residue.

The amount of residue used in the bayer process is:

pretreatment residue C: 114g/L of the suspension; pretreatment residue D: 128g/L suspension.

After the action of the residue under the above conditions, the red mud (i.e. the solid residue resulting from the action) is rapidly separated from the liquid, then washed, dried and analyzed. The concentrate was stabilized and analyzed. Table 7 shows the analysis of the slurry (results obtained by X-ray fluorescence).

The X-ray diffraction pattern of the slurry resulting from the action of the Bayer liquor on the pretreated residue C shows the presence of a corundum phase and the presence of a cancrinite phase (Na) resulting from the insolubilization of sodium hydroxide in the form of sodium aluminosilicate₂O x Al₂O₃ x 2SiO₂x 1/3(2 NaCl)). Traces of garnets (a calcium aluminate) were also observed, as well as quartz, portlandite and goethite. No garnet phase, such as tricalcium aluminate (3CaO xAl), was detected₂O₃ x 6H₂O) or guava (Ca)₂Al₂(SiO₄))。

The X-ray diffraction pattern of the slurry resulting from the action of the Bayer liquor on the pretreated residue C shows the presence of corundum phase, cancrinite and sodalite (Na)₂O x Al₂O₃ x 2SiO₂x 1/3(2 NaCl)); in addition, potash feldspar is accompanied by another portlandite type calcium phase. Hematite and anatase were also found.

The bayer liquor resulting from the digestion of the pre-treatment residue was also analyzed.

The iron (13 mg/L; 6mg/L) and silica (1 g/L; 0.7g/L) contents are those typically found in aluminates from bauxite processed by the Bayer process.

Calculating the dissolution rate of alumina after the Bayer process: al of residue C₂O₃-SiO₂Yield of (5) was 90.23%, and Al of residue D₂O₃-SiO₂The yield of (D) was 97.41%. Al of residue C₂O₃The total yield of (1) is 75.04%, and Al of residue D₂O₃The total yield of (a) was 67.28%.

The yields obtained indicate that the residue improved by the pretreatment including calcination and leaching can be advantageously used for the production of alumina by the bayer process. The alumina (in combination with silica) in the bauxite physically enriched residue is mostly extracted, thereby increasing the amount of extractable alumina. In the case of residue C, the amount of extractable alumina increased from 90% to 95% of the total alumina, and in the case of residue D from 46% to 84%. The bayer effect makes it possible to dissolve approximately 79 to 80% of the alumina that can be extracted from the residues C and D.

3.2Conclusion

The above examples show that the process according to the invention allows the use of bauxite physical enrichment residues by modifying their crystallization characteristics, so that they can be used by bayer processes adapted to the pretreatment residue. Of particular note are:

for the washing residue C:

heat treatment of the crude washing residue C can convert total kaolinite and remove 98% of the bound silica;

the presence of 7% quartz, the majority entering the solution (90%) and combining to form insoluble sodium aluminosilicates, reduces the efficiency of the bayer process;

the use of such pretreated residues for the production of alumina reduces the amount of residue produced by the bayer process by 16% compared to the use of non-pretreated residues.

For flotation residue D:

coarse flotation residue D, containing 55% kaolinite and a very small amount of quartz, is more difficult to handle due to its high content of phyllosilicates (63%);

the need to add heat treatment to avoid the generation of phases under leaching conditions that may be recombined with sodium hydroxide;

70% of the mixed silica was removed. The efficiency of the bayer process is limited by the formation of sodium aluminosilicates, mainly from residual dissolved and recombined silicates;

the amount of lime required to desilicate the leachate (taking into account the amount of insoluble dissolved silica) is high. Calcined and leached residue produced in an amount of 1.07t/t silicate;

the final residue from the bayer process will represent 59% of the mass of the initial pre-treatment residue used;

thermal treatment makes it possible to remove the impregnation residues and the organic substances coming from the flotation aids.

The bayer process performance for these pre-treated residues is consistent with that obtained with natural diaspore type bauxite having equivalent a/S ratio.

In the presence of a sufficient amount of lime, the corundum is dissolved at a rate of between 75% and 85% during the action of the bayer process.

Claims

1. A process for the manufacture of alumina trihydrate or alumina from a physically enriched residue of bauxite, which has been pretreated by a process comprising calcination (2020) and leaching (2030), resulting in a residue known as pretreated residue,

the method comprises the following steps:

(a) treating (2120) the pre-treated residue with an aqueous sodium hydroxide solution at a temperature of at least 100 ℃, the aqueous sodium hydroxide solution having a concentration of 100g Na₂O/L and 220gNa₂Between O/L, preferably 140g Na₂O/L and 200g Na₂Between O/L, more preferably 155gNa₂O/L and 190g Na₂Between O/L, and even more preferably 160g Na₂O/L and 180gNa₂Between O/L;

(b) separating (2130) the solid residue from the liquid phase;

(c) crystallizing (2160) aluminum trihydrate by adding seed crystals (2170);

(d) separating (2180) the crystalline aluminum trihydrate from the liquid phase,

(e) optionally, calcining (2194) the aluminum trihydrate obtained in step (d) to obtain aluminum oxide (2196).

2. The process according to claim 1, wherein the temperature of step (a) is between 150 ℃ and 350 ℃, preferably between 200 ℃ and 300 ℃, more preferably between 220 ℃ and 280 ℃, and even more preferably between 250 ℃ and 270 ℃.

3. Method according to claim 1 or 2, characterized in that the Al of the pre-treated residue₂O₃/SiO₂The mass ratio is greater than 3.0, preferably greater than 4.0, and even more preferably greater than 5.0.

4. A method according to any one of claims 1 to 3, characterized in that the alumina content of the pre-treatment residue is greater than 50%, preferably greater than 55%, and even more preferably greater than 60% by mass.

5. A method according to any one of claims 1 to 4, characterized in that the silica content of the pretreated residue is less than 20%, preferably less than 15%, and even more preferably less than 14% by mass.

6. A method according to any one of claims 1 to 5, characterized in that the loss on ignition of the pre-treated residue is less than 5%, preferably less than 3%, and even more preferably less than 2.5%.

7. The method as claimed in any one of claims 1 to 6, characterized in that the pretreated residue has a diaspore content of less than 3 mass% and a kaolinite content of less than 3 mass%.

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

9. The method according to any one of claims 1 to 8, characterized in that the pre-treatment residue has been pre-treated by calcination (2020) at a temperature between 980 ℃ and 1050 ℃, preferably between 990 ℃ and 1040 ℃, and even more preferably between 1000 ℃ and 1040 ℃.

10. The method according to any one of claims 1 to 9, characterized in that the pre-treated residue has been pre-treated by leaching (2030) with an aqueous sodium hydroxide solution.

11. The method according to any one of claims 1 to 10, comprising the steps of:

o-calcining (2010),

o leaching with aqueous sodium hydroxide (2030),

o separating (2040) solids from the aqueous leach phase, said separated solids being said pre-treatment residue,

(iii) treating the pre-treated residue by a method according to any one of claims 1 to 10.

12. Method according to any of claims 1 to 10, characterized in that the residue is Al-pretreated₂O₃/SiO₂The ratio is between 1 and 7, preferably between 1 and 5.5, even more preferably between 1 and 4, and most preferably between 1 and 3.

13. An apparatus for implementing the method of any one of claims 11 or 12, comprising:

-a unit for pretreating bauxite mineral matter beneficiation residue by calcination and leaching, so that the residue can be converted into pretreated residue; and

-a unit for producing alumina from the pretreated residue, implementing the process according to any one of claims 1 to 9,

the method is characterized in that:

-the pre-processing unit comprises

o at least one calciner for calcining the residue,

o at least one leaching unit for leaching the calcined residue with an aqueous sodium hydroxide solution, known as "leachate", and

-the unit for preparing alumina from the pre-treatment residue comprises

o at least one chamber for treating said pre-treated residue with an aqueous solution of sodium hydroxide, called "Bayer liquor", at a temperature of at least 100 ℃,

14. The apparatus of claim 13 wherein the bayer liquor from the solid liquid separation unit for separating the crystallized aluminum trihydrate from the bayer liquor is recycled to the digestion step (2120).