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
  • C01F7/06—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
  • C01F7/0626—Processes making use of tube digestion only
• • 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

Definitions

• This invention relates to a process for extracting or digesting gibbsite (alumina trihydrate) from various forms of bauxite, such as those containing: (1) mainly gibbsitic alumina; (2) gibbsitic and boehmitic alumina; or
• the present invention relates to a process and apparatus where the extraction into caustic-aluminate liquor of alumina trihydrate from a bauxite can be carried out rapidly into caustic-aluminate liquor in a reaction tube interposed between vessel for slurrying the gibbsite-containing bauxite and a solid- liquid separator for separating insoluble red mud solids or red mud solids plus undissolved boehmitic and/or diasporic alumina from the liquor containing the dissolved trihydrate or gibbsite.
• a slurry of a bauxite containing both gibbsite and boehmite may be pressure digested in caustic soda solution or spent caustic-aluminate solution being recycled in the Bayer process to dissolve the aluminum hydroxides as sodium aluminate while leaving undissolved red mud solids.
• the resulting caustic solution, containing an increased concentration of sodium aluminate, is called pregnant sodium aluminate containing liquor, or pregnant liquor.
• red mud solids can be more easily and advantageously separated from the pregnant sodium aluminate containing liquor using separation techniques operating at above atmospheric pressures, corresponding to temperatures above the atmospheric boiling point of the digested slurry.
• a portion of the silica content of the bauxite may dissolve in the liquor to form soluble sodium silicates, some of which may slowly complex with sodium aluminate in solution to form hydrated sodium aluminum silicates, also known as the "desilication product.”
• Precipitation of desilication product while relatively easily accomplished, may cause an appreciable loss of expensive caustic soda.
• the dissolved silica contained in the desilication product may be removed in accordance with the disclosure of U.S Patent No. 5,118,484 by contacting the pregnant Bayer process liquor with porous agglomerates of desilication product bonded together by a polymer resin selected to withstand the extreme conditions found in Bayer process slurries or other forms of desilication product seed.
• the present invention provides a process for extrac ⁇ ting gibbsite from a bauxite containing gibbsitic alumina, comprising: (i) forming a stream of ground bauxite containing gibbsitic alumina with a small portion of Bayer process spent caustic aluminate liquor used for digestion; (ii) mixing the stream with the remainder of the spent liquor stream previously preheated to form a preheated slurry- liquor mixture; (iii) passing the preheated liquor-slurry mixture through one or more parallel reaction tubes sized such that the slurry remains in the reaction tube for a residence time just sufficient to extract essentially all of the gibbsite from the slurry and no more than about four minutes; (iv) separating red mud solids, still containing undissolved boehmite and/or diaspore alumina, from the pregnant liquor in a solid-liquid separator operating at substantially the same temperature and pressure as in the reaction tube; and
• the temperature of the preheated mixture in the reaction tube ranges from about 120 ⁇ C to about 200"C, and the preheated slurry remains in the reaction tube for a mean residence time ranging from several seconds to about 3-4 minutes.
• the solid-liquid separator separates red mud solids from pregnant Bayer process liquor in a pressure decanter or other suitable means of pressure solid/liquid separation, preferably at a temperature above the atmospheric boiling temperature of the liquor in the slurry, and preferably at a temperature sufficiently high that the liquor phase of the slurry is not saturated with respect to gibbsitic alumina.
• the temperature and pressure of the pregnant liquor in the pressure decanter is substantially the same as that of the mixture in the reaction tube apart from pressure drops due to the friction of the flowing slurry.
• the seeded post- desilication is carried out at similar conditions, or, alternatively, the temperature and pressure of the pregnant liquor may be reduced to its atmospheric pressure and boiling point prior to desilicating the pregnant liquor by a seeded post-desilication operation.
• the invention also provides an apparatus for extracting gibbsite from gibbsite containing bauxite which comprises: a vessel for mixing a slurry of ground gibbsite containing bauxite in a minimum portion of the spent caustic aluminate liquor being passed to the digester with the remainder of the spent liquor previously preheated to a temperature comparable to or slightly above the digestion temperature to form a slurry having a predetermined temperature and liquid-solid ratio; an elongated reaction tube, in fluid communication with the mixing vessel, for retaining the slurry in the reaction tube at a predetermined pressure and temperature for an average mean residence time of up to about four minutes; a pressure decanter, in fluid communication and sealed relationship with the reaction tube, for separating red mud particles from pregnant liquor; and a vessel for contacting the pregnant liquor with Bayer process desilication product so as to carry out a seeded post-.
• the apparatus also includes means for preheating spent Bayer process liquor to maintain the slurry in the digester at a temperature in the range of about 120°C to about 200°C.
• FIG. 1 is a schematic flow chart of the improved process of the present invention
• FIG. 2 is a schematic flow chart of the Bayer process improved in accordance with the present invention
• FIG. 3 is graphical representation of plant-scale demonstration of the present invention in which the ratio of the concentration of dissolved A1 2 0 3 (expressed in grams/liter) to the concentration of total caustic (expressed as g/1 Na 2 C0 3 ) is plotted against up to 2 minutes residence time in the improved digestion apparatus of the present invention, plus a further residence time of 2 to 18 minutes in a conventional low temperature digester. Best Modes For Carrying Out the Invention
• Extracting the gibbsite from bauxite in the Bayer process usually involves grinding the bauxite in a side stream of recycled spent caustic aluminate liquor.
• the ground bauxite may be mixed with the remainder of the liquor being recirculated to the digester, and the combined stream of liquor and bauxite heated to the digester temperature (typically - 100-160°C) (so-called "single-streaming”) .
• the remainder of the liquor not used for grinding the bauxite may be preheated to a temperature somewhat exceeding the digester tempera ⁇ ture and can then be mixed with the colder concentrated bauxite/liquor slurry so that the final temperature is close to the required digest temperature (so-called "double-streaming") .
• the temperature may be further adjusted by addition of heat to the digester itself.
• the heated mixed slurry is held in agitated autoclaves or cascades of autoclaves for mean residence times from about 20 minutes to over 1 hour.
• the digested slurry is cooled to near the atmospheric boiling temperature.
• the red mud is then separated, typically by sedimentation, using flocculants if required, and/or filtration.
• the red mud slurry separated in this way which contains supersaturated aluminate liquor, is washed to recover soluble values and is disposed of or further processed.
• the clarified liquor is cooled, diluted, and seeded with gibbsite particles onto which a significant part of the dissolved alumina precipitates as gibbsite.
• the coarser part (larger particles) of the precipitate is calcined to produce alumina, while the rest is recirculated as seed.
• the final caustic-aluminate mother liquor (spent liquor) is reconditioned and recirculated to digestion.
• the disadvantages of the conventional technology for extracting gibbsite from bauxite are well known in the art, and include: a) the liquor productivity in digestion (kg of A1 2 0 3 produced per m 3 liquor circulated) is relatively low, since productivity is often restricted by the instability of the liquor at the red mud separation stage, so that the full dissolving power of the liquor for gibbsitic alumina at the digester conditions cannot be completely used; (b) the low liquor productivity combined with the long residence time requires large holding vessels capable of operating at 2-8 atmospheres pressure, during digestion; (c) the bulky equipment becomes scaled on all its wetted surfaces with tenacious scales of desilication product and/or boehmite (which is relatively insoluble at gibbsite digestion conditions) , requiring periodic manual and/or chemical descaling; (d) the pressurized digesters require mixers to maintain the slurry in suspension, involving expensive and difficult to maintain shaft seals; and (e) the much lower solubility of boehmite than gibbsite
• boehmite contains any boehmite, and "gibbsitic" bauxites typically contain up to 2%, (expressed as Al 2 0 3 ) , it will not be extracted. This will serve as a seed material for the reprecipitation of gibbsitic alumina dissolved into solution as additional boehmite. This material gets lost along with the original boehmite with the red mud. In many plants carrying out gibbsitic digests only, such "reversion" losses of boehmite represent the largest loss of alumina in the process, and add significantly to the consumption of bauxite and generation of red mud per ton of alumina produced. This reversion of boehmite increases rapidly with increasing digestion time due to the self-catalyzing nature of the reversion process.
• the inventive process can be understood as essentially eliminating the gibbsite digester as such, and substitutes a preheated liquor/bauxite slurry mixer and pipeline conveying the combined mixed slurry during a minimum mean residence time directly to a pressure mud separator, followed by seeded post-desilication after red mud separation.
• Tests have demonstrated that the process has several advantages.
• the very short time of residence required only to achieve essentially complete gibbsite dissolution, which is known to occur rapidly means that the digester volume is minimal (a few percent of that of the conventional digester) thereby greatly reducing the cost of the pressure equipment required. See G.D. Fulford, Light Metals 1985 (ed. H.O.
• the very short digestion time coupled with the decreased red mud separation time will reduce to almost zero the amount of gibbsite to boehmite reversion which can occur, even in cases where the "gibbsitic" bauxite contains small amounts of seeding boehmite. As explained above, this in turn will reduce the bauxite consumption and red mud generation per ton of alumina produced.
• the short digest time makes it possible, with some gibbsitic bauxites, to extract essentially all of the gibbsitic alumina content of the bauxite while only dissolving a part of the clay mineral, thereby reducing caustic soda loss.
• the digestion time By careful control of the digestion time, one can maximize the fraction of the clay-mineral silica which does dissolve which passes to the post- desilication step. A minimum amount of the desilication product will then fall out with the red mud, and most will form a separate stream of relatively pure and highly concentrated desilication product from the seeded post- desilication operation. This will simplify the possible recovery of bound caustic soda and alumina values from the desilication product.
• the process of the present invention can be used for: 1) plants with single gibbsite digest operations; 2) plants with double gibbsite digest operations; and
• the improved process of the present invention as it may be used in the two-stream mode for digestion of gibbsitic bauxite may be understood by reference to Fig. 1.
• Gibbsitic bauxite 13 is first wet-ground with a small portion 10a of a stream of spent, unheated Bayer process liquor 10 in a grinder 12.
• the remainder 10b of the spent liquor stream 10 is preheated to digestion temperature or above this temperature in preheater 11 and the preheated liquor 14 is fed into a slurry mixer 16 together with a gibbsitic bauxite/liquor slurry 15 from grinder 12.
• the weight of the solids in the slurry 15 from the grinding mill 12 preferably ranges from 30 to 70 percent by weight, although typically, solids content approaches about 50 percent.
• the mixed preheated slurry 17 will typically have a solids content in the order of 10 percent w/w.
• the spent liquor should be preheated so that when the combined mixed slurry 17 is formed, it has a temperature ranging from about 110 to 170 ⁇ C, with a temperature of approximately 130-145°C being preferred.
• the combined slurry 17 undergoes digestion in an elongated reaction tube 18, which is preferably an insulated tube or pipe.
• This reaction tube is unheated through its external surfaces, although it may in some instances include means for final temperature adjustment by injection of live steam.
• the average residence time within the reaction tube 18 ranges from a few seconds to about 4 minutes (depending mostly upon the temperature, the caustic concentration of the slurry, and the rate at which the gibbsite fraction of the particular bauxite extracts into the liquor (easily determined from laboratory tests) ) .
• the digestion in the reaction tube 18 is completed, at least 90% and preferably more than 98% of the gibbsitic alumina has been extracted.
• the dissolved silica content of the liquor would typically be quite high (perhaps several g/L Si0 2 ) , since there has not been sufficient time for the desilication reaction to proceed to completion.
• the digested slurry 19 proceeds to a red mud separation step 20 (Fig. 1) carried out at approximately digestion conditions, i.e. at approximately the same temperature and pressure as was present in the reaction tube or pipeline 18.
• a red mud separation step 20 (Fig. 1) carried out at approximately digestion conditions, i.e. at approximately the same temperature and pressure as was present in the reaction tube or pipeline 18.
• the red mud separation takes place in a pressure solid/liquid separator 20 which rapidly separates the red mud particles 21 from the digested slurry essentially at the digestion temperature and pressure, preferably at a temperature above the atmospheric boiling temperature of the liquor phase of the slurry.
• the liquor phase 22 from the red mud separation step 20 undergoes a seeded post-desilication step 23 (FIG.
• the total average residence or digestion time between the point at which the bauxite slurry and the preheated spent liquor are mixed and the entry to the pressure mud separation device should be kept to the minimum required for dissolving the gibbsite.
• the actual time needed will vary depending upon the type of bauxite and the bauxite grind, as well as the digest temperature, the caustic concentration of the extracting liquor, and the closeness of approach required to the gibbsite solubility value at the end of the digest.
• the mean residence time should fall between a few seconds and 3-4 minutes.
• the improved process for digestion of gibbsite contained in bauxite may take place in an apparatus such as the one schematically diagrammed in Fig. 2.
• the apparatus, generally designated by the reference numeral 30 includes a conduit 32 through which ground bauxite is fed and mixed with a preheated spent liquor stream 34 (or a preheated stream of liquor containing intermediate alumina to caustic soda ratio in the case when the gibbsite digest forms the first part of a counter-current double digest) .
• the combined preheated bauxite slurry is fed through an elongated reaction tube 36.
• the elongated reaction tube 36 is preferably an unheated but insulated pipeline connecting the mixing device in a sealed relationship with the pressure red mud separation device 39.
• final adjustment of the digest temperature may be effected by the injection of live steam into the reaction tube at suitable points, such as steam injectors 35 and 37 in FIG. 2.
• the diameter of the reactor tube 36 and number of tubes are not critical provided the tube or tubes are sized and arranged so as to achieve the velocity of flow normally required for conveying the solids fraction of a Bayer process slurry.
• the reactiont tube may range in diameter from about 100 to about 500 or more millimeters in diameter with about 200 millimeters presently preferred. Its length depends upon several factors both related and unrelated to slurry concentration type, temperature and pressure.
• the length of tube is not critical, and while 40 meters appears optimal for digesting bauxite having a high percentage of gibbsitic alumina, the tube may range in length from a few meters to more than 100 meters as needed to maintain a residence time of several seconds to 3 to 4 minutes.
• the single tube may be replaced by a plurality of tubes.
• the length and diameter will have to be determined empirically in order to balance space constraints, necessary velocity and turbulence to avoid settling of the slurry and too-rapid scaling of the reaction tube 36 with the objective of keeping the total average digestion time long enough to permit substantially complete extraction of the gibbsite. It should be noted that some scaling of the reactor walls is tolerable in this invention, since heat is not transferred through the walls for purposes of heating the slurry.
• the shape of the tubing used as the pipeline or tubular reactor can vary and may, for example, be fashioned as a serpentine or helically coiled circuit to conserve space and reduce heat loss.
• the reacted slurry 28 leaves the reaction tube 36 and enters a pressure red mud separation device 39 (such as a pressure decanter) which rapidly separates red mud particles from digested slurry at or above the digestion temperature and pressure (or close thereto) .
• the separated red mud flows through pipeline 40 where it may undergo further processing or disposal.
• the pregnant liquor flows through pipeline 42 and enters a seeded post silication device 44.
• Desilication seed one example of which is described in U.S. Patent No. 5,118,484, enters through pipeline 46 and excess product is withdrawn through line 50.
• the desilication step may take place at digestion temperature or, alternatively, after cooling the liquor to about the atmospheric boiling temperature.
• the method and apparatus of the present invention simplifies post-desilication and yields a greater fraction of the total desilication product (containing chemically bound alumina and soda as well as silica) as a product separate from the red mud, thereby making it simpler to recover the bound values from it by known means, for example, a lime soda sinter step.
• the desilicated liquor continues via pipeline 48 to cooling, polish filtration, if required, and precipitation tanks (not shown) .
• EXAMPLE 1 Plant-scale tests using 40 m 3 /h of a 50% w/w slurry in spent liquor of ground bauxite (from the Cape York area of Queensland, Australia), containing 28.5% gibbsitic A1 2 0 3 on the dry bauxite basis, at approximately 60°C, was mixed with 100 m 3 /h of preheated spent caustic aluminate liquor (caustic concentration about 255 g/L Na 2 C0 3 , dissolved alumina concentration -82 g/L A1 2 0 3 ) at approximately 162°C, leading to a temperature of about 130°C after mixing and the gibbsite dissolution was complete.
• the mixing was accomplished by injecting the bauxite slurry and preheated spent liquor into an insulated cylindrical mixing pot of volume 3.4 m 3 , which was connected to the inlet of an agitated digester vessel by approximately 40 m of insulated pipe of diameter 200 mm.
• the mean residence time of the mixed slurry was therefore min 3 -.4.nr3 x in the pot and, at a mean velocity of -74.4 m/min in the
• FIG. 3 shows that the dissolved silica value found at this point varied from 4.0 to 4.5 kg/m 3 Si0 2 , which is 7.5 to 8.5 times the equilibrium silica value in this liquor, since the relatively slow desilication reaction had not yet had time to remove the dissolved silica down to its equilibrium value.

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• 1994
  • 1994-01-21 EP EP94904925A patent/EP0681560A1/de not_active Withdrawn
  • 1994-01-21 AU AU58766/94A patent/AU5876694A/en not_active Abandoned
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