Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F7/00—Compounds of aluminium
  • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
  • C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
  • C01F7/14—Aluminium oxide or hydroxide from alkali metal aluminates
  • C01F7/144—Aluminium oxide or hydroxide from alkali metal aluminates from aqueous aluminate solutions by precipitation due to cooling, e.g. as part of the Bayer process
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F7/00—Compounds of aluminium
  • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
  • C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
  • C01F7/06—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom by treating aluminous minerals or waste-like raw materials with alkali hydroxide, e.g. leaching of bauxite according to the Bayer process
  • C01F7/062—Digestion
  • C01F7/0633—Digestion characterised by the use of additives
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B33/00—Silicon; Compounds thereof
  • C01B33/20—Silicates
  • C01B33/32—Alkali metal silicates
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F7/00—Compounds of aluminium
  • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
  • C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
  • C01F7/06—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom by treating aluminous minerals or waste-like raw materials with alkali hydroxide, e.g. leaching of bauxite according to the Bayer process
  • C01F7/0646—Separation of the insoluble residue, e.g. of red mud
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F7/00—Compounds of aluminium
  • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
  • C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
  • C01F7/14—Aluminium oxide or hydroxide from alkali metal aluminates
  • C01F7/144—Aluminium oxide or hydroxide from alkali metal aluminates from aqueous aluminate solutions by precipitation due to cooling, e.g. as part of the Bayer process
  • C01F7/147—Apparatus for precipitation

Definitions

• the invention relates to the field of ore processing, and more particularly the physical and chemical treatments of bauxite.
• the invention particularly relates to a method of heat treatment and chemical treatments of bauxite low alumina / silica mass ratio.
• the silicon ore is first depleted by thermal pretreatment followed by leaching, and then this pre-processed ore is used in the Bayer process to extract the aluminum in the form of aluminum trihydrate, which can be transformed into alumina.
• Aluminum is the third most abundant chemical element in the Earth's crust, after oxygen and silicon. Associated with oxygen it is found in a very large number of rocks.
• the main industrial ore of aluminum is bauxite, discovered in 1821 in the village Les Baux (France) by the geologist Pierre Berthier.
• Bauxite represents a complex mixture of oxides of aluminum, iron and silicon which may include various impurities such as titanium, calcium, magnesium.
• bauxite is an ore comprising mainly three aluminum minerals, namely gibbsite, boehmite and diaspore, mixed with lower amounts of iron minerals, namely goethite and hematite (which gives bauxite its characteristic color), as well as aluminosilicates (kaolinite, illite, ...) and titanium minerals (anatase, rutile, ilmenite).
• the main industrial process to extract aluminum (as oxide) of the bauxite is the Bayer process, developed at the end of I 9 th century. It essentially comprises two stages: a first step of leaching of the ore under pressure by a soda solution (see patent DE 43 977 of August 3, 1888), and a second step of precipitation of pure hydrated alumina from the solution of sodium aluminate thus obtained, by seeding with crystals of hydrated alumina (see patent DE 65604 of November 3, 1892).
• This precipitation alumina, hydrated, can then be subjected to heat treatments to dehydrate it; this heat treatment also determines the structure and morphology of the alumina obtained, with a view to its use (in the Hall-Héroult process for producing aluminum by electrochemical reduction of the alumina in molten salt, or as technical alumina, in particular in the ceramics industry). More specifically, the Bayer process consists mainly of a selective attack (digestion) of alumina hydrates contained in bauxite by a solution (called "liquor") of hot caustic soda, which is recycled.
• the soda solution enriched with sodium aluminate is cooled then decomposed (crystallization phase) to precipitate and extract the alumina trihydrate (Al2O3 -3H2O); the latter is then washed and then calcined at high temperature to give the alumina (Al2O3).
• composition of bauxite depends on its geographical origin. This composition variation concerns both its content of main elements (Al, O, Si), its content of impurities, and its mineralogical structure.
• lateral bauxites on alumino-silicate geological substratum from mines in Guinea or Australia generally have a high gibbsite content (which is a trihydrate) and a lower silicon content than karst bauxite (on carbonate geological substratum) from mines in Iran, Ukraine, Azerbaijan and Turkey in which aluminum is predominantly in the form of boehmite and diaspore (two monohydrate modifications).
• a simple parameter for representing the quality of a bauxite is the ratio of alumina to silica, abbreviated as "A / S ratio".
• a / S ratio the ratio of alumina to silica
• bauxites in Guinea generally have AI2O3 / S1O2 ratios in the order of 20 or more, and bauxites in Western Australia have ratios greater than 15.
• bauxites in northern Queensland aluminum is present mainly in the form of boehmite and gibbsite.
• the aluminum content is however not the only criterion: it is still necessary that this aluminum is in a chemical and crystallographic form that can be extracted from bauxite by the Bayer process. It is known that the traditional Bayer process does not make it possible to solubilize the aluminum contained in the aluminosilicates: this part of the aluminum is lost in red mud. Moreover, it is known that the aluminosilicates contained in the red mud can carry away part of the sodium hydroxide and thus increase the overall consumption of soda from the Bayer process; this is described in the Xiaofeng Zhu et al. publication "Basic Research on Calcification Transfomation, Process of Low Grade Bauxite", published in Light Metals 2013, p.
• TMS 239-244
• bauxite is traded on a global market and is often transported thousands of miles to where it is used.
• some countries prefer to limit their supply to the world market in favor of the use of their domestic mining resources, even if they are of poorer quality. This is particularly the case of China.
• China the largest aluminum producer in the world, has deposits and bauxite mines that do not have (or more) as high a A / S ratio as Australian bauxites, for example. More precisely, in China, deposits of alumina with a low silicon content (A / S> 8 or even> 6) become rare, whereas bauxite with a high silicon content exists in considerable quantities.
• silicon is largely present in the form of kaolinite, an aluminosilicate that also contains some of the aluminum present in bauxite; they also include a small fraction of quartz (a silicate) and muscovite (another aluminosilicate).
• quartz a silicate
• muscovite another aluminosilicate
• thermochemical activation process The idea of a bauxite thermochemical activation process (called “roast-leach process”) has been described by Smith and Xu-Parker ("Options for processing high silica bauxites", ICSOBA work Vol 35 (39), 184-192 (2010)). This process was initially developed for clay-like raw materials (see US 2,939,764) and illite (see T Jiang et al., “Desilication from illite by thermochemical activation", Trans., Nonferrous Met.Soc., China, Vol 14 (5). ), pp.
• the residue (calcined and leached bauxite) is then introduced into the traditional Bayer process.
• This method initially has the disadvantage of requiring additional thermal energy for the calcination of bauxite, to arrive at a calcined and leached bauxite which has characteristics comparable to that of a good quality bauxite. This overconsumption of energy can put a brake on the development of the process.
• the process requires an additional raw material, namely lime, to precipitate the silicate resulting from the solubilization of the amorphous silica formed during the calcination of bauxite.
• the Roast-Leach process gave rise to a fairly large scientific literature, which focused on the calcination step, with divergent indications as to the optimum calcination temperature.
• the maximum value of the calcining temperature seems to be given by mullite formation: According to Xu et al.
• the decomposition of meta-kaolinite into amorphous silica and gamma-alumina begins at 990 ° C, but at around 1 ° C, the temperature and temperature of the alkali-leaching process (Light Metals TMS 2002). 100 ° C these two phases begin to react to form mullite, insoluble in sodium under the conditions of the traditional Bayer process; a temperature between 1000 ° C and 1050 ° C is recommended.
• the problem that the present invention seeks to solve is to develop an improved and economically viable method or process for using bauxites with a low alumina content (A / S ⁇ 5 and preferentially ⁇ 4 and even more preferentially ⁇ 3) in a process.
• Bayer process type Objects of the invention
• a bauxite treatment process which comprises pretreatment of bauxite, which improves its ability to be used as raw material in the Bayer process, known per se, and by adapting the process. Bayer at this pretreated bauxite.
• Said pretreatment of the bauxite comprises a first pretreatment step, which is a physical pretreatment, namely thermal.
• This thermal pretreatment is intended to cause a chemical and crystallographic modification of the bauxite (or at least some of these mineralogical phases which constitute it).
• This first step of pretreatment of the bauxite is advantageously carried out on a milled bauxite. It leads to a modified bauxite which can then be introduced into the Bayer process, or be subjected to a second chemical pretreatment step.
• said thermal pretreatment is carried out at a temperature and for a time such that at least a portion of the silicates present in the bauxite is converted into amorphous silica.
• this temperature is advantageously between 1000 ° C. and 1050 ° C., preferably between 1015 ° C. and 1030 ° C., and even more preferentially between 1015 ° C. and 1025 ° C.
• this temperature is below about 40 ° C and is between about 960 ° C and about 1000 ° C and preferably between 970 ° C and 990 ° C.
• This process can be implemented partly on the industrial site of bauxite production or entirely on the industrial site where the Bayer process is installed. It requires dedicated equipment, namely an oven. This process is advantageously carried out on ground bauxite.
• the bauxite treatment process comprises a second pretreatment step, which is a chemical step. It includes soda leaching of modified bauxite. During this step, and under the appropriate conditions of temperature, residence time, sodium concentration and solid / liquid ratio, the amorphous silica obtained during the calcination is dissolved while the alumina passes very little in solution. .
• This leaching step must be carried out on a milled bauxite, and for this reason it is advantageous that the grinding is done upstream of the heat pretreatment step.
• the grinding process and target grain size can be similar to those used for the traditional Bayer process. It is also possible to regrind the treated bauxite before its introduction into the Bayer process.
• pretreated bauxite which is chemically and mineralogically distinct from natural bauxite.
• a simple parameter that expresses this peculiarity of pretreated bauxite is its loss to fire; this loss on ignition is much lower (typically by a factor of ten to twenty) than that of a natural bauxite (not pretreated).
• a first object of the invention is a process for producing alumina or alumina trihydrate from pretreated bauxite by a process comprising calcination and leaching, said pretreated bauxite being characterized in that it has a loss on ignition less than 2.5% by weight, preferably less than 2.0%, and even more preferably less than 1.5%.
• this pretreated bauxite is also characterized by the absence of diaspore and the presence of amorphous silica.
• the method comprises the steps of:
• step (e) calcining the aluminum trihydrate obtained in step (d) to obtain alumina (2196).
• step (e) is necessary.
• the temperature in step (a) is between 150 ° C and 350 ° C, preferably between 200 ° C and 300 ° C, more preferably between 220 ° C and 280 ° C, and even more preferably between 250 ° C and 280 ° C. ° C and 270 ° C.
• Said pretreated bauxite advantageously has a mass ratio of Al 2 O 3 / SiO 2 greater than 8, and preferably greater than 9, and even more preferably greater than 10. Its mass content of alumina is advantageously greater than 60%, preferably greater than 65%, and even more preferably greater than 70%. Its silica mass content is less than 12%, preferably less than 10%, and even more preferably less than 8%.
• the liquid phase resulting from step (d) is reintroduced into the aqueous sodium hydroxide solution used in step (a).
• said pretreated bauxite has been pretreated by calcination at a temperature between about 920 ° C and about 1200 ° C.
• This temperature is preferably between about 950 ° C and about 1070 ° C, and more preferably between about 1000 ° C and about 1050 ° C, especially in the case of diasporic bauxites; for boehmitic bauxites, a lower calcination temperature of from about 950 ° C to about 1100 ° C, more preferably from about 960 ° C to about 1000 ° C, and more preferably from about 970 ° C to about 100 ° C is preferred. 990 ° C. This calcination leads to a chemical and crystallographic transformation of bauxite.
• a major (and often all) fraction of diaspore (which is the form in which the vast majority (and often almost all) of alumina is found in low A / S bauxites) is transformed into alpha alumina.
• This transformation is accompanied by the departure of certain volatile materials present in the bauxite or formed during said chemical and crystallographic transformation.
• Loss on ignition is a readily determinable parameter that synthetically expresses the state of this chemical and crystallographic transformation during calcination.
• the calcined bauxite has a better solubility of aluminum contained in particular in aluminosilicates under the usual conditions of the bayerite digestion step of the Bayer process, and a better solubility of silicon under milder conditions than those of the step of digestion of the Bayer process.
• the treatment of calcined bauxite by leaching with an aqueous solution of sodium hydroxide under milder reaction conditions than those of the digestion step of the Bayer process makes it possible to solubilize the silica.
• pretreated bauxite has a diaspore mass ratio that is much lower than in bauxite; after calcination, this level is preferably less than 5%, more preferably less than 3%, even more preferably less than 2%, and optimally less than 1%.
• this level is preferably less than 5%, more preferably less than 3%, even more preferably less than 2%, and optimally less than 1%.
• the mass content of kaolinite in pretreated bauxite is preferentially less than 4%, more preferably less than 3%, even more preferably less than 2%, and optimally less than 1%.
• a preferred pretreated bauxite for use in the process according to the invention has a loss on ignition of less than 2%, a diaspore content of less than 3% and a kaolinite content of less than 3%, these percentages being mass percentages. More preferably, this pretreated bauxite has a loss on ignition of less than 2%, a diaspore content of less than 2% and a kaolinite content of less than 2%, and even more preferably a loss on ignition of less than 1.5%. %, a diaspore level of less than 1% and a kaolinite content of less than 2%.
• Another subject of the invention is a process for producing alumina trihydrate or alumina from a bauxite comprising the following steps:
• Said bauxite capable, after pretreatment by calcination and leaching with sodium hydroxide, to enter the process according to the invention, advantageously has an Al 2 O 3 / SiO 2 ratio of between 1 and 8, preferably of between 1 and 7, and even more preferably between 1 and 5.5, more preferably between 1 and 4, and even between 1 and 3 or between 2 and 3.
• the pretreatment of the bauxite according to the invention leads to a significant increase in the Al 2 O 3 / SiO 2 ratio, typically by a factor of two to three.
• the process may not be economically viable, since the fact of using a pretreated bauxite rather than a natural bauxite does not lead to an additional yield of alumina sufficiently interesting, and the potential for reducing the consumption of soda is limited.
• Another subject of the invention is an alumina that can be obtained by the process according to the invention.
• Yet another object of the invention is an installation for carrying out the process according to the invention, comprising: a unit for pretreatment of bauxite by calcination and lixiviation, making it possible to convert a bauxite into pretreated bauxite; and
• said pretreatment unit comprises
• At least one calcination furnace for calcining bauxite at least one calcination furnace for calcining bauxite
• leaching unit for leaching the calcined bauxite with an aqueous solution of sodium hydroxide (so-called "leachate solution"), and o at least one solid-liquid separation unit for separating the calcined and leached bauxite from said leaching solution ;
• said alumina production unit from said pretreated bauxite comprises at least one enclosure (such as an autoclave or tubular device) for treating the pretreated bauxite with an aqueous solution of sodium hydroxide (so-called "Bayer liquor”) at a temperature of at least 100 ° C,
• At least one solid-liquid separation unit for separating the solid residue (called "red mud") from said Bayer liquor;
• At least one solid-liquid separation unit for separating crystallized aluminum trihydrate from said Bayer liquor
• said Bayer liquor from said solid-liquid separation unit used to separate the crystallized aluminum trihydrate from the liquid phase is recirculated to the digestion step.
• Figure 1 shows a simplified diagram of the Bayer process according to the state of the art.
• Figure 2 shows a simplified diagram of an embodiment of the method according to the invention.
• Figures 3 and 4 refer to Example 3 and show thermogravimetric and thermodifferential analysis curves (ATG - ATD) of bauxite samples, the rise in temperature corresponds to the calcination to obtain calcined bauxite.
• the curve that refers to the right scale represents the mass loss (PM) per minute ( Figure 3) and the heat flux ( Figure 4).
• Figure 5 shows the loss on ignition (measured after calcination at 1060 ° C) of a high silica diasporic bauxite which has been calcined at different temperatures.
• the invention will be explained in detail with respect to the Bayer process according to the state of the art which is shown in FIG.
• the bauxite from a bauxite mine is milled (step 1100) in the presence of a liquid phase, which is sodium aluminate, as will be explained in greater detail below.
• the grinding is intended to increase the specific surface area of the bauxite accessible to the action of the liquid phase during the attack with a view to the digestion of bauxite. It is typically a grain size of a few hundred ⁇ .
• the grinding is done with the addition of lime (step 1102), in the form of milk or in solid form.
• Lime has a triple action: (i) During the digestion of bauxite lime decreases the consumption of soda as it promotes the precipitation of soluble silicates in the form of calcium silico-aluminates rather than in the form of silico-aluminates sodium (which would otherwise carry a portion of sodium sodium, more expensive than lime); (ii) lime promotes the dissolution of aluminum and improves the extraction efficiency of alumina during digestion; and (iii) the lime improves the settling of the sludge after attack, because it favors the transformation of the goethite, difficult to decantable and filterable, in hematite, better crystallized.
• the milled bauxite is then etched with an aqueous solution of sodium hydroxide (step 1110) under pressure and at high temperature in autoclaves or tubular exchangers.
• This attack leads to the partial digestion of bauxite (step 1120), more precisely, it is the soluble part of the aluminum minerals (alumina in particular, whether present in the form of monohydrate or trihydrate) which forms aluminate ions.
• the digestion is carried out at a temperature between 250 ° C and 270 ° C in closed autoclaves or tubular exchangers.
• Said aqueous sodium hydroxide solution is in practice an aqueous solution of sodium aluminate.
• a sodium concentration of between 235 g Na 2 O / L and 245 g Na 2 O 5 is typically used.
• the person skilled in the art knows how to fine-tune the parameters of this step, in particular the temperature, the residence time and the concentration of the soda, with the composition of the bauxite; the same applies to the amount of lime added in step 1102.
• bauxites with a high content of monohydrate aluminas need more digestion temperatures.
• high bauxites with a high trihydrate content gibbsite).
• the temperature range between 250 ° C and 270 ° C ensures that all the soluble alumina contained in the bauxite (including the diaspore fraction, which is the most easily soluble among the aluminum oxides, and whose content can be very variable) is digested.
• karst bauxites which are the main field of application of the present invention, require this temperature range.
• Some factories using karst bauxites are even designed to work at temperatures up to 280 ° C, so that they can adapt to the use of bauxites of very specific composition.
• the bauxite can be brought into contact with the preheated liquor (so-called double flow process), or the bauxite suspension in the Bayer liquor is constituted before being heated (so-called single-flow process).
• the digestion step 1120 is carried out in two stages, each operated at a different temperature, in order to first dissolve the easily soluble fractions and then, at a higher temperature, the solid residue of the first stage. This double-digestion variant can save energy, but it requires a larger investment and complicates the process.
• red mud When the suspension is relaxed (step 1124), in successive expansion stages, part of the water evaporates (self-evaporation).
• the residue (called “red mud") is separated from the liquid phase (liquor); flocculating agents are added which increase the rate of phase separation and improve the clarification of the liquors (ie the residual amount of dry matter in the liquid).
• the solid residue is called “red mud”; it contains all crystalline phases from bauxite not reactive to digestion (1120) and those formed during the Bayer cycle.
• Red mud is recovered (step 1140) and washed with water (step 1142) to recover a maximum of the liquor; this washing is generally done with raw water and against the current (to minimize the amount of water used); it is followed by a decantation and / or filtration step (not shown in the figure).
• Red mud is a powdery residue which is not easy to recycle, and which often ends up in specific storage facilities.
• the liquid phase (“L") resulting from the phase separation step 1130 is a sodium aluminate solution.
• the aluminum trihydrate is crystallized (step 1170) by cooling the aluminate and adding seeds (also called “primers”) of aluminum trihydrate (step 1160). This stage of crystallization is called in the profession the "decomposition”; its duration is of the order of 40 hours.
• the dilution with cold water (step 1150) reuses the red mud wash waters.
• This step 1170 uses a certain know-how, known to those skilled in the art, in order to adapt the numerous parameters of the process (saturation of the liquor at the entry of the decomposition, concentration of Na2O and impurities of different natures, head temperatures and end of decomposition, primer surface, crystallization technology, grading classification) at best to the nature of the desired alumina product; the physicochemical phenomena involved mainly concern nucleation (spontaneous formation of fine particles within a suspension), agglomeration of fine particles, granulometric classification by cycloning and / or decantation.
• the precipitated trihydrate is separated by decantation and filtration (step 1180) using various known technologies; it is recovered (step 1190). A significant portion of the trihydrate must be recycled to the decomposition step 1160, the remainder is dried (step 1192) and calcined (step 1194) to alumina. The latter is stored (step 1196) for routing to a consumer site.
• the drying step (step 1192) is typically carried out as the first calcination step (step 1194) which takes place in several stages. During this rise in temperature, the impregnating water is first removed from about 100 ° C., and then the water forming the trihydrate (at around 1000 ° C.); the rise in temperature is then continued to obtain the desired crystalline structure.
• the liquid phase resulting from the phase separation step 1180 is an aqueous sodium hydroxide solution, more dilute than that used in step 1110 because of the various water inflows into the stream (red mud wash water (step 1142) and trihydrate, dilution water (step 1150)). For this reason it must be concentrated by evaporation of water (step 1210) to be recycled (step 1220) into the soda solution used for the digestion step (step 1120). It is also reused (step 1222) in the wet milling step of bauxite (step 1100).
• the trihydrate obtained in step 1190 can be washed before drying; this wash water can be reused in the washing of red sludge in step 1142 (not shown in the figure).
• sodium hydroxide is consumed during the treatment of bauxite to produce alumina. More specifically, this consumption is related to three mechanisms: (i) the formation of insoluble sodium silico-aluminate phases during the attack (digestion step 1120); (ii) the residual soda entrained with the sludge (1140) despite being washed (step 1142); (iii) co-precipitation with alumina during the crystallization phase (1170). These losses must be offset by the addition of new soda (step 1110). Whenever possible, all washing liquid phases containing sodium hydroxide (including the chemical cleaning of tanks and pipes) are recycled in the Bayer liquor. 2.
• FIG. 2 One embodiment of the process according to the invention is illustrated in FIG. 2. It comprises a pretreatment of the bauxite. Pretreated bauxite is introduced into the Bayer process.
• the Bayer process steps denoted 11xx and 12xx in FIG. 1 are designated in FIG. 2 by the markers 21 xx and 22xx, whereas the preprocessing steps carry the marks 20xx.
• Pretreated bauxite is a product that does not exist as such in nature, it is necessarily a product resulting from an industrial process, namely the pretreatment process. Its chemical composition differs from that of natural bauxite, from which it has two essential characteristics: it has a higher A / S ratio (because it contains fewer silicates), and it has only very little water of crystallization. . Moreover, its mineralurgical composition is different, following the transformations that it undergoes during the various stages of pretreatment, as will be explained in greater detail below.
• diasporic bauxites containing silica mainly in the form of kaolinite the main differences concern the dehydration of diaspore and its transformation into alumina, mainly alpha, as well as kaolinite which after dehydration is transformed into metakaolinite (as explained in section 2.3). below), allowing the solubilization of silica with sodium hydroxide.
• the very low water content of crystallization distinguishes the chemical composition of pretreated bauxite from that of natural bauxites with a similar A / S ratio.
• the loss of water of crystallization is the main parameter that enters the loss (mass) to fire.
• a natural diasporic bauxite generally has a loss on ignition greater than about 10%
• a pretreated bauxite according to the invention has a loss on ignition of less than 2.5%, preferably less than 2%. , 0%, and even more preferably less than 1, 5%.
• Loss on ignition is a parameter known to those skilled in the art; further explanations are given below in section 2.3.
• the bauxite from a bauxite mine is milled (step 2000) after the addition of water (step 2002), filtered (step 2004), and the solid residue after filtration (step 2004) is calcined (step 2010).
• this intermediate here "calcined bauxite”.
• An aqueous soda solution is added (step 2020) and the calcined bauxite is leached (step 2030).
• the phase separation step 2040
• the solid phase is recovered which is here called “leached calcined bauxite” or "pretreated bauxite” and introduced into the Bayer process; depending on the grain size obtained during grinding in step 2000 it may be necessary to regrind (step 2100, not shown in Figure 2).
• the liquid phase from the phase separation in step 2040 is lime treated to precipitate the silicates (step 2050).
• a white sludge step 2070
• the liquid phase resulting from the phase separation in step 2060 is an aqueous solution of sodium hydroxide; it is recovered (step 2080) and recycled partially into the attack solution of the Bayer process.
• Advantageously lime (2052) is introduced in the form of milk of lime.
• this pretreatment method can comprise numerous variants.
• the phase separation step 2060 may be followed by an additional step of filtering the liquid phase (step not shown in FIG. 2, designated here 2062). It may comprise a washing of white sludge (step not shown in FIG. 2, designated here 2064) after the phase separation step 2060, the washing water being recycled at step 1150.
• Bauxite can also be milled dry, and in this case it goes directly to the calcination step (2010).
• pretreated bauxite in the Bayer process can be done in the same plant, ie with the same equipment, and according to the same process scheme, as the implementation of untreated bauxite. (The only exception is the evaporation step 2210 which may be omitted in certain variants of the process according to the invention). However, if the same operating parameters are used for the implementation of the pretreated bauxite (for example, the duration, the temperature and / or the concentration of the soda), a result different from that obtained with a bauxite is obtained. not treated.
• the operating parameters are modified with respect to a usual operation of the Bayer process.
• the calcination temperature (step 2010) of bauxite has a strong influence on the extraction efficiency of the alumina of the leached bauxite. According to the invention this temperature must be greater than 980 ° C for a diasporic bauxite.
• the kaolinite is activated and is not completely transformed, it reacts with leaching (step 2030) to give an insoluble compound of the zeolite type.
• a calcination temperature above 990 ° C is preferred. According to an advantageous embodiment, it is greater than 1000 ° C.
• a temperature between 1010 ° C and 1035 ° C the transformation of kaolinite is total; a temperature of between 1020 ° C and 1030 ° C is preferred.
• the calcination process is followed by leaching which will be explained below.
• the improved pretreated bauxite resulting from this calcination - leaching process can be introduced as such in the Bayer process.
• this method is modified. Specifically, some operational parameters are changed, which substantially reduces, among other things, energy consumption. 2.2 Specific embodiments
• step 2000 the grinding can be carried out in a cylindrical mill containing balls or steel bars.
• the quantity of water (step 2002) can be of the order of 0.7 m 3 per ton of bauxite, with a bauxite load of the order of 1000 kg per m 3 . This gives a suspension of water and bauxite which can be separated by filtration on a filter press (step 2004). Residual impregnation of water of the order of 10% by weight is acceptable.
• the grain size referred to grinding may be the same as that in the case of the traditional Bayer process, namely a few hundred ⁇ .
• the calcination in step 2010 can be carried out in a rotary or static type furnace.
• the progressive heating makes it possible to eliminate the impregnating water of the ore, then the water of constitution, of the crystalline phases present in the bauxite, then to carry out the transformation of these phases, at the temperatures indicated above. Under these conditions, we observe that:
• Silica which was present in the form of silicates, is mostly converted into amorphous silica
• the diaspore and boehmite phases are converted into "alpha" type alumina
• Iron that was present in the form of goethite is, after calcination, converted into hematite (Fe 2 O 5);
• the phases containing carbon, carbonates and sulfur are thermally dissociated in the form mainly of CO2 and SO2 for the volatile part. Leaching (step 2030)
• the sodium content of the liquid phase can be between about 70 g NaOH / L and about 160 g NaOH / L, preferably between about 90 g / L and about 150 g NaOH / L, and even more preferably between about 1 10 g / L and about 140 g NaOH / L.
• a content of 129 g NaOH IL has been used successfully.
• This solution can be obtained from a mixture of recycled soda and 50% sodium hydroxide solution, the quantities of which are adjusted to obtain the concentration necessary for leaching.
• a residence time ie a contact time between the solid phase and the liquid phase
• the temperature of the aqueous sodium hydroxide solution is typically between 80 ° C and 120 ° C; if the temperature is too low the silica dissolves badly, if the temperature is too high one tends to dissolve alumina.
• the calcined bauxite and the sodium hydroxide solution (2010) may be introduced into a stirred reactor vessel so as to obtain an initial suspension containing approximately 80 kg / m 3 of solid.
• a reaction temperature of about 100 ° C is suitable; the residence time at the reaction temperature may be of the order of 45 minutes.
• Phase separation (step 2040)
• the phase separation in step 2040 can be carried out by filtration on a filter-type filter fed by the suspension from the leaching reactor (2030).
• the solid residue is "calcined leached bauxite” or "pretreated bauxite; a residual impregnation of leach liquor of the order of 10% by weight is acceptable.
• the liquid phase is a liquor charged with dissolved silica after leaching (2030); it is refined by adding lime (2052).
• the raw material advantageously consists of quicklime (CaO) or milk of lime (Ca (OH) 2). It is preferred to use quicklime with a fine particle size, containing at least 85% of CaO; typically it contains between 85% and 95% of CaO.
• the milk of lime can be manufactured by extinction of this lime (of the order of 100 kg of CaO / m 3 ) with hot water in a stirred tank reactor.
• the silicates precipitation step makes it possible to form an insoluble calcium silicate.
• Precipitation of the silica can be done in a stirred reactor tank in the presence of quicklime or milk of lime (100 g CaO / L) at 100 ° C for 2 hours.
• the stoichiometric ratio CaO: SiO 2 is advantageously between 1, 1 and 1.5.
• a suspension is typically obtained which contains, at the end of the operation, of the order of 35 kg / m 3 to 50 kg / m 3 of solid, preferably between 39 kg / m 3 and 46 kg / m 3 of solid.
• the phase separation (step 2060) can be advantageously by decantation of the suspension.
• the clear liquid phase (“Over-flow”) is advantageously recycled to the soda circuit of the process, preferably in part to leaching (2030), and partly upstream of the digestion step of the Bayer process (2120).
• the thickened suspension made of calcium silicate (typically 600 to 700 kg of solid / m 3 ), called white sludge (2070), is extracted from the clarifier. It can feed a filter, type band filter, on which operates a methodical washing (step 2072) with water to reduce the concentration of the impregnating liquor.
• the washed white sludge may have a residual impregnation of diluted liquor of the order of 10%; the sodium hydroxide concentration of this impregnating liquor is typically of the order of 6 to 10 g NaOH IL.
• the white sludge is composed of a calcium silicate which is close to that of Tobermorite (approximately Ca4.35Si5.5iAlo, 50i6 (OH) 2 x 4 H2O). It can be directed to intermediate storage awaiting recovery.
• the wash water of silicate sludge (white sludge 2070) recovered at the end of step 2072 can join the circuit of the liquid phase 2080 obtained in step 2060 to be used in the leaching step (step 2030) and in the Bayer process (step 2120). In the latter case it requires a readjustment of its sodium content (step 2110) which will have become lower than the initial content in step 2030 (for example 129 g NaOH / l). This readjustment is carried out with the addition of an aliquot of 50% sodium hydroxide solution.
• the pretreatment according to the invention generates a certain loss of soda, by occlusion of sodium in the precipitated silicate and by impregnation of white sludge. This loss must be offset by the addition of sodium hydroxide solution, typically at 50% (step 2110).
• the modified Bayer process according to the invention consumes less sodium than the traditional Bayer process, reduced to the ton of alumina produced.
• the Bayer liquor is recirculated (step 2200); it consists of a liquor mixture from the filtration step of the trihydrate (step 2180) having or not a concentration by evaporation of water (step 2210) and addition of sodium hydroxide solution (step 2110).
• loss on ignition is a parameter that is an integral part of the usual characterization of a bauxite; this value, expressed in mass percents, appears on the certificate of analysis that accompanies any delivery of bauxite intended for the Bayer process. It is determined as a rule by calcination at 1060 ° C for 2 hours, after pre-drying at 105 ° C. The calcination of bauxite always leads to a net loss of mass, which is caused by the departure of volatile matter, even though there may be oxidation reactions which, taken in isolation, lead to an increase in mass.
• the loss on ignition mainly corresponds to the elimination of the water of constitution (ie water molecules which are integrated in the crystallographic structure), of the carbon dioxide resulting from organic matter and mineral carbonates, and of certain other volatile compounds, especially sulfur oxides.
• the loss of fire from bauxite depends on its chemical and mineralogical composition.
• its order of magnitude is typically between 10% and 30%. It can be determined by simple weighing before and after calcination under the conditions indicated. Differential thermogravimetry can also be used, which also makes it possible to characterize the mineral species present in bauxite.
• the loss on fire of diasporic bauxites with a high silica content which constitute a raw material that can be used in the context of the present invention to prepare the so-called pretreated bauxite, is typically between 10% and 18%, and more often between 12% and 15%, but these empirical figures do not limit the scope of the present invention.
• FIG. 5 shows the loss on ignition (determined after calcination at 1060 ° C. for 2 hours) of a diasporic bauxite calcined according to the invention at various temperatures ranging from 980 ° C. to 1030 ° C.
• the loss on ignition after calcination at 1030 ° C is extremely low (0, 10%), or in other words: when this material is heated above 1030 ° C and up to 1060 ° C, the amount of volatile material that is released is extremely low.
• Al 2 Si 2 O 5 (OI-1) 4 is decomposed into Al 2 O 3 X 2 SiO 2 (meta-kaolinite) + H 2 O; - Exothermic dissociation between 900 ° C and 1000 ° C: Al 2 0 3 x 2 Si0 2 (meta-kaolinite) decomposes into 2 Al 2 O 3 X 3 SiO 2 (pseudo-mullite) + S1O2 (amorphous) + ⁇ AI2O3.
• phase separation makes it possible, preferably after filtration, to separate the solid from the liquor in order to be able to use the pretreated bauxite in the digestion step ( step 2120) of the Bayer process.
• the inventors made a number of observations which led them to modify certain steps of the Bayer process; this modification is an essential feature of the present invention. Digestion (step 2120)
• Digestion consists of solubilizing the aluminous phases contained in the pretreated bauxite in a sodium hydroxide solution.
• the alpha alumina contained in the pretreated bauxite which was generated during the calcination, is solubilized by the Bayer liquor at high temperature, along with the pre-existing soluble alumina.
• This step can be carried out under conditions of temperature and pressure similar to those of the traditional Bayer process, namely: a temperature typically between 250 ° C. and 270 ° C. in closed autoclaves or in a pressurized tubular system (approximately 50 bar at 60 bars).
• the heating is advantageously by gradually increasing the temperature to the reaction temperature.
• the residence time at the reaction temperature is advantageously between 30 min and 60 min, preferably between 30 min and 50 min, and even more preferably between 35 min and 45 min.
• a significantly lower soda concentration can be used for the digestion of pretreated bauxite than that used for normal bauxite in the traditional Bayer process. More particularly, this concentration is between 140 g Na 2 O / L and 200 g Na 2 O / L, preferably between 155 g Na 2 O / L and 190 g Na 2 O / L, and even more preferably between 160 g Na 2 O / L and 180 g Na 2 O / L. This concentration is advantageously monitored continuously by measuring the electrical conductivity of the liquor; it can also be the subject of a chemical analysis in the laboratory.
• the inventors have found that despite a greater circulating flow rate of the etching liquor (12, 11 m 3 / t against 8.52 m 3 / t) due to a lower concentration of caustic soda (162 g / l against 240 g / l) of the digestion liquor, the extraction yields of the alumina remain very high; they are greater than 96% on silica-less alumina by adapting the adjustment parameters of the workshop for example, the saturation of the liquor (concentration of alumina and RP of lime addition (8 to 10.4%) and temperature (260 ° C).
• the process according to the invention uses a Bayer liquor with a soda concentration significantly lower than the traditional Bayer process, the quantity of water to be evaporated is much lower. Moreover, the pretreated bauxite generates less red mud (2140) than the majority of natural bauxites, which reduces the amount of wash water (2150) sludge necessary, knowing that this washing water loaded with soda will be introduced in the circuit of the liquor Bayer. In some cases evaporation upon expansion of the autoclave (step 2124) to digestion (2120) is sufficient to maintain the concentration of the recycled aluminate liquor (2220); the evaporation step 2210 can then be omitted. The evaporation step 2210 consumes thermal energy and requires a significant investment in evaporators; the fact of being able to minimize or even eliminate this step is of great economic interest.
• the suspension is diluted with an aluminate liquor from the first red sludge washing stage at step 2140.
• This dilution is regulated according to the entry of wash water. It makes it possible to reach a liquor concentration compatible with solid-liquid separation and crystallization at step 2160.
• the decantation is carried out in a device called “decanter", large diameter tank (most of the time with a flat or conical bottom) provided with a slow stirring allowing the separation. Decantation is optimized by the use of additives called flocculant to increase the rate of sedimentation of solid particles.
• flocculant additives to increase the rate of sedimentation of solid particles.
• the absence of certain phases, such as goethite, which was transformed during the calcination of bauxite in step 2010 and which is known to hinder flocculation makes it possible to reduce the amount of flocculant. employee.
• the thickened suspension (Under-flow) is sent to the first washing stage.
• the clarified liquor (Over-flow) is sent to the filtration called "safety" which aims to remove very fine sludge particles to ensure a liquor free of impurities to crystallization.
• the washing of red sludge in step 2140 is preferably carried out against the current; the wash water is introduced to the last stage of the scrubber chain.
• the scrubber chain can be completed by filtering sludge from the last scrubber using a filter press.
• the use of flocculant improves sedimentation to ensure better sludge washing.
• the small amount of red sludge generated (typically reduced by about 60%) by the digestion (2120) of pretreated bauxite requires a lower amount of water (typically reduced by about 50%) than necessary to wash the sludge generated by the digestion of untreated bauxite.
• the amount of washing water and the tonnage of sludge are the following: ⁇ 3.04 m 3 / t of water per 1 063 t / t of sludge after washing in the case of a bauxite treated by the process according to the invention,
• the inventors have found that with the pretreated bauxite by the process according to the invention, the judicious adjustment of the main parameters of the Bayer process, in particular to the attack (residence time, saturation of the liquors, amount of lime added, etc.) makes it possible to maintain excellent solubilization yields of alumina (greater than 96% on alumina less silica) with ethanol liquors. low concentration of caustic soda (of the order of 160 to 170 g / L for 238 g / L to 240 g / L for conventional attacks). These results and improved efficiency of sludge washing can drive the Bayer liquor cycle with low caustic concentration, resulting in significant energy savings and maintenance costs at the shop floor. evaporation. By eliminating the evaporation step 2210, the economy of the investment in an evaporation plant is also reduced in the case of a new production line.
• the process according to the invention leads to a significant reduction in the amount of sludge, even taking into account the fact that the desilication step (2050) (also called “desilication”) by precipitation of the silicates generates specific sludge ("sludge"). "white”) that do not appear in the traditional Bayer process with untreated bauxites: in the process according to the invention it is found that the reduction in the amount of red sludge is much greater than the amount of white sludge. Moreover, the subsequent fate (and the possible valorization in usable products) of these types of sludge is not the same.
• the process according to the invention reduces the quantity of sludge to be decanted (steps 2040, 2060 and 2130), typically from 10 to 40%, and preferably from 20% to 40%. For red mud this reduction can reach 65% (step 2130).
• these sludges have a better settling ability because of the transformation, during the calcination phase of the ore, from goethite to hematite.
• the consumption of soda during the Bayer process is related to the formation of solid compounds including sodium (silicoaluminate, sodium crystallized with alumina) on the one hand and loss in liquid form of soda (red mud impregnation liquor). , and trihydrate) on the other hand.
• the calcination-leaching pretreatment process also generates losses by sodium occlusion (occlusion in the silicate) and by losses under liquid form of soda (impregnation of white sludge). All of these losses are offset by an addition of 50% sodium hydroxide solution.
• the energy consumption of the Bayer process for diasporic bauxites of low Al / Si ratio is of the order of 8 GJ per tonne of alumina obtained, without taking into account the calcination of the trihydrate (step 1194, 2194); insofar as the energy consumption of the calcination of the trihydrate does not depend on the origin of the bauxite, only the process up to the trihydrate is compared here (1190, 2190).
• this consumption is reduced by 1. 9 GJ / t thanks to the suppression of the evaporation step 2210.
• the process according to the invention consumes less water and less soda than the traditional Bayer process applied to a poor karst bauxite.
• the consumption of soda passes from 490 kg of NaOH to 104 kg of NaOH per tonne of alumina produced.
• the consumption of lime doubles, from about 360 kg per ton of alumina to about 800 kg per ton of alumina, but the cost of lime amounts to about 10% of the cost of NaOH.
• This investment consists in adding a pre-treatment unit to an existing plant that operates the Bayer process: no modification of the equipment is necessary within this existing plant (apart from the integration of the Bayer liquor streams between the two "pre-treatment” units and "Bayer” which relates to piping): the modification that the present invention brings to the Bayer process is significant, but it only concerns the operating parameters of the Bayer process, and not the industrial equipment. So we see that economically the process is largely winning. Moreover, the significant reduction in the amount of red sludge, which tends to have a negative economic value, also reduces the cost of their reprocessing and storage.
• white sludge mainly silicates
• red sludge they contain less heavy metals and other potentially toxic substances (if they were to pass into solution) than red sludge; their economic value is not negative.
• the mineralogy of calcium silicate in the form of Tobermorite suggests some applications especially in the field of construction.
• the greatest advantage of the process according to the invention is certainly the possibility of increasing the valuable deposits in bauxite in the accounting (microeconomic or macroeconomic), by making it possible to use mineral resources which can not be used with the processes according to the invention. state of the art under competitive economic conditions.
• the ability of some mills to use bauxites from geographically close mines results in associated transportation cost savings.
• this pretreated bauxite can come either from a separate bauxite pretreatment plant (for example, installed near a bauxite mine, in order to save on the cost of transporting bauxite), or from an integrated plant (pretreatment unit + Bayer unit) which is overcapacity in pretreated bauxite.
• This first embodiment with a separate pretreatment unit is however not preferred because the recovery of the sodium-laden liquid phase resulting from the leaching step 2040 and the washing of the white sludge 2072 can not in this case be done. by recirculation in the Bayer liquor. 4.
• the method according to the invention has many advantages. Its main advantage is to allow the implementation, as part of the Bayer process, low Ai / Si bauxites that can not be used according to the state of the art, or with a low yield only, and with a cost significantly higher compared to bauxites with a higher Ai / Si ratio. Another advantage is that the pretreatment according to the invention eliminates not only the silicon, but also almost all the organic carbon and a large part of the sulfur naturally contained in the bauxite. It is known that organic carbon accumulates in the sodium aluminate liquor and some can precipitate as oxalate on the trihydrate.
• red sludge is reduced (up to 60%). This has two advantages: at the level of the phase separation, and at the level of their ultimate treatment. More specifically, the process according to the invention makes it possible to reduce the quantity of red sludge; it generates a new type of residue, white sludge, but the sum of white and red sludge is reduced (up to 25%) compared to red sludge according to the traditional Bayer process.
• the red sludge generated in the process according to the invention show a better settling ability (because of the transformation during the calcination (step 2010) of goethite in hematite), which makes it possible to reduce the quantity of flocculating agent added to the suspensions, to facilitate the management of the decantation workshop, to reduce the cost of its maintenance, and to reduce the consumption of electrical energy.
• Another advantage is the possible decrease in hard deposition of aluminosilicates on the surfaces of the different equipment used in the digestion (step 2120) and downstream of the digestion step; this hard deposit tends to hinder the heat exchange and must be removed from time to time in specific maintenance operations.
• Yet another advantage is that the residual iron and silica content of the alumina obtained by the process according to the invention is particularly low.
• Another advantage, already mentioned above, is the reduction in the amount of water to be evaporated in the water evaporation step (step 2210), which can be suppressed in many cases. This contributes significantly to energy saving.
• the method according to the invention comprises an additional step of calcination (step 2010) which consumes thermal energy and soda.
• step 2010 this soda can be recycled largely in the Bayer process and the energy consumption of the calcination stage is almost compensated by the saving made on the Bayer process.
• the process according to the invention can advantageously be used with a pretreated bauxite obtained from natural bauxites having an A / S ratio of between 1 and 8 and preferably between 1, 5 and 7.
• a value of 7 or 8 the profit in terms of additional alumina made available by the process of pretreatment of bauxite by calcination and leaching becomes lower, and the advantage of reducing the consumption of soda is more limited because there is less silica likely to lead to soda.
• This upper threshold depends on certain technical and economic parameters that may vary according to the economic data.
• the mineral resource is mainly kaolinite, which has other technical applications.
• Natural bauxites rarely have an A / S ratio of less than about 2.5 or 2, and the preferred lower limit of the process according to the invention is the use of a pretreated bauxite obtained from a naturally occurring bauxite having A / S ratio equal to 2.
• This crude ore contains a relatively large amount of alumina, but is also rich in silicon (low A / S ratio, about 4.6), and the silica is mainly present in the form of kaolinite (88%), the remains (12%) being in the form of muscovite.
• Samples of 200 g of this bauxite were calcined at 980 ° C or 1030 ° C.
• the calcination was done in a muffle furnace preheated to 200 ° C - 250 ° C, with a rise in temperature as quickly as possible.
• the calcination time was 30 minutes at the target temperature (980 ° C or 1030 ° C).
• the bauxite was cooled in a desiccator. Its chemical composition was analyzed by X-ray fluorescence, its structure by X-ray diffraction, its content of volatile matter (in English "loss on ignition", abbreviated LOI) by weighing before and after heating at 1060 ° C.
• LOI loss on ignition
• the lixiviation was carried out on a suspension at a rate of 90 g / l of calcined bauxite, with a pure sodium hydroxide solution at 100 g / l of Na 2 O, for a volume of suspension of 500 ml.
• the suspension thus obtained was kept for one hour at 100 ° C., and was then filtered on a Millipore membrane filter (5 ⁇ ).
• the filtrate was stored in an oven at 90 ° C, an aliquot was taken for analysis.
• the leached bauxite was washed and dried.
• the aqueous attack phase was an industrial Bayer liquor of composition (in g / L):
• Na 2 0 ctq refers to the useful fraction ("caustic") of soda
• Na 2 0 cbte refers to the fraction of Na 2 0 which corresponds to carbonate residues
• Silicon was precipitated by adding lime milk; the amount of lime added varied between 2.5% and 4%. This is described in greater detail in Example 2 below.
• the bauxite load was adjusted to obtain a RP (alumina / soda ash) value at the end of the attack, which varied between 1.00 and 1.28.
• RP alumina / soda ash
• Precipitation of the silica was carried out with lime milk at 100 g / L CaO and with solid CaO, at a temperature of 100 ° C for 2 hours.
• the milk of lime was prepared with stirring at 70 ° C for 90 min and then at 85 ° C for 60 min.
• the precipitation of the silica was carried out on a volume of 300 ml of the first hot filtration solution of the leaching test of a W201 burned bauxite.
• lime milk precipitates silica better (87%) than solid lime (74%), but it also precipitates a little more aluminum (49% instead of 37% for solid CaO). It is also noted that lime milk hardly affects the calcium content of the solution.
• the characterization of the precipitates by X-ray diffraction shows that they are composed of the same crystallized phases, the Tobermorite formed during the precipitation of Si by the portlandite, and the calcite which was already present in the lime used.
• Precipitation of the silica was carried out with lime milk at 100 g / L CaO (lime from a Chinese plant) at a temperature of 98 ° C for 2 hours. Table 5 below shows the chemical composition of this lime.
• Table 10 shows the analysis of precipitated silicate (white sludge).
• the precipitation rate is between 87 and 90%, for precipitation with lime milk, and 74% when using solid lime.
• the insolubilization of soda is estimated at 0.0017 points per point of dissolved silica.
• Example 3 Xiao Yi Bauxite A Xiao Yi bauxite powder (Shanxi, China) was supplied, the chemical analysis of which is detailed in Table 11 below.
• This crude ore has a composition substantially different from that of Jiaokou bauxite.
• its aluminum content is lower, and its silicon content is higher: the A / S ratio is very low, about 2.7; silica is mainly present in the form of kaolinite.
• the calcined bauxite was then leached at 100 ° C. for 45 minutes in a solution of 100 g / l of Na 2 O (corresponding to 130 g / l of NaOH), with a solids concentration of 80 g / l.
• This leaching was carried out in a jacketed reactor heated with a thermostatic oil bath.
• the suspension was filtered on a slow filtration paper filter (Whatman 589/3) in a Buchner funnel mounted on a vial connected to a vacuum pump. A first filtration was carried out hot, then two other filtrations were carried out after repulping the cake with deionized water at room temperature. The cake of the second filtration was washed on a filter with ethanol.
• the high silica content therefore comes mainly from kaolinite and incidentally from quartz and other phyllosilicates ( micas, palygorskite).
• Thermogravimetric analysis (ATG) coupled with differential thermal analysis (DTA) essentially shows the dehydroxylation of diaspore and kaolinite (see Figures 3 and 4).
• ATG Thermogravimetric analysis
• DTA differential thermal analysis
• At around 900 ° C we observed an aspect of the curve that suggests the hypothesis of crystallization of a newly formed compound that could be mullite.
• Table 14 compares the chemical analyzes of the original bauxite, after calcination and after leaching:
• the leached bauxite was ground (all passing at 300 ⁇ ) and dried in an oven at 110 ° C.
• the attack was carried out in a 150 ml autoclave heated at 260 ° C. for 40 minutes.
• the lime added to the attack came from a Chinese alumina plant; it contains 86.02% CaO.
• the lime was quenched and was added as milk. Different amounts of lime have been tried.
• the suspension was filtered.
• the solid was washed and prepared for chemical and crystallographic analyzes (loss on ignition, X-ray fluorescence and X-ray diffraction).
• the liquor was analyzed with Methrom (Na2O, Al2O3, carbonate).
• the observed dispersion is probably partly due to the difference in the amount of lime added to the attack. This important parameter has an influence on the yield, the yield curves as a function of the weight ratio of alumina / caustic soda, and the quantity of insoluble soda.
• Example 3 On the basis of the excellent results of Example 3, the inventors have sought to further improve their process. Given that the process of pretreatment of bauxite (Roast-Leach process) strongly reduces the mass of bauxite at the entrance of the Bayer process and thus red sludge at the outlet (with mass of alumina produced constant) one can think that the washing of sludge will be more effective in the context of the process according to the invention. If an identical washing efficiency is maintained, it is possible to reduce the amount of net sludge washing water, thereby reducing the total amount of water to be evaporated in the Bayer process. Thus, the energy consumption is greatly reduced at this stage of the process.
• Roast-Leach process strongly reduces the mass of bauxite at the entrance of the Bayer process and thus red sludge at the outlet (with mass of alumina produced constant)
• Example 4 the Xiao Yi bauxite used in Example 3 was treated by the same method as that described in Example 3, but its digestion in the Bayer process was carried out at reduced sodium concentration: lowered the Na2O concentration from 238 g / L to 171.5 g / L.
• Table 17 The results are summarized in Table 17 below.
• solubilization yield of alumina remains very high despite a considerable drop in the caustic soda concentration of the attack liquor.
• Wanji bauxite has been supplied (Henan province).
• the content of Si0 2 therefore comes essentially from muscovite and quartz, incidentally from kaolinite.
• X-ray diffraction characterization revealed the disappearance of kaolinite, the conversion of diaspore to corundum (0AI2O3), siderite and goethite to hematite.
• the other phases, muscovite (residual structure), quartz, magnetite, anatase and rutile are always present.
• the detection limit of this X-ray characterization is less than 1% by mass.
• the leaching in sodium medium makes it possible to lower the S102 content from 12.58 to 8.58%, ie a decrease of 4 points, and a change from the alumina / silica ratio of 4.0 to 7.3 (see Table Z2). above).
• Another bauxite was supplied from Shanxi (China). It has an alumina / silica ratio of 1.94. X-ray diffraction analysis indicates that it is a diasporic bauxite whose silicates consist essentially of kaolinite, a low proportion of muscovite, and quartz. Iron is present essentially in the form of goethite.
• This bauxite was calcined for 30 minutes at 1030 ° C.
• the leaching of this calcined bauxite was carried out at 100 ° C. with sodium hydroxide at 100 g Na 2 O l / liter for 45 minutes.
• the analyzes of the raw, calcined (1030 ° C) and leached bauxite are reported in Table 19 below:
• the X-ray diffraction characterization reveals the disappearance of kaolinite, and the appearance of a silicic phase, mullite.
• a diffractogram bulge is observed in a zone 2 ⁇ between 18 ° and 20 °, reflecting the formation of a poorly crystallized silica phase. The diaspore has disappeared. Corundum is present. Goethite has been transformed into hematite.

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