Coarsening of Partially Calcined Alumina Dust

This invention is directed to the coarsening of finely divided alumina dust. In one aspect this invention is directed to the coarsening of finely divided alumina dust in the presence of Bayer pregnant liquor under conditions such that concomitant with the precipitation of alumina trihydrate from the Bayer pregnant liquor the finely divided alumina dust added to the Bayer pregnant liquor is coarsened.

In the production of alumina via the Bayer process involving calcining of the alumina trihydrate precipitated in the Bayer process, a substantial amount of alumina dust is produced. The recovery and/or elimination of this alumina dust is of considerable environmental and economic interest. Alumina dust in the production of alumina via the Bayer process is usually recovered by mechanical and/or electrostatic devices, such as filters, cyclone dust separators and electrostatic precipitators. The resultant recovered dust is usually partially calcined and has a very fine particle size, particularly the alumina dust recovered from the electrostatic precipitators which might have a particle size such that at least about 80-90% by weight of the partially calcined alumina dust is smaller than about 44 microns (325 mesh), more or less.

One technique for the disposal of the fine alumina dust generated in the production of alumina from alumina trihydrate produced vis the Bayer process involves blending the alumina dust with the alumina product stream leaving the calcining apparatus wherein alumina trihydrate is calcined to alumina. The amount of fine alumina dust blended to the product alumina stream usually amounts to about 2-3% by weight of the total product flow. The fine alumina dust in the alumina product tends to cause environmental problems and to eventually result in the loss of useful alumina material since the fine dust escapes from the alumina product during material transfer and handling between the alumina plant and the aluminum-producing plant and even at the aluminum production cells.

Another technique employed for disposal, actually coarsening of the recovered alumina dust, involves agglomerating the alumina dust in Bayer pregnant liquor under relatively short-cycle batch precipitation conditions. Under these conditions, however, due to the extreme fineness of the alumina dust, essentially all of the alumina dust having a particle size smaller than 44.mu., the coarsening or agglomeration of the alumina dust is very slow. In fact, it has been observed that alumina dust, particularly the very fine, partially calcined alumina dust recovered from the electrostatic precipitators, may yield a product possessing a greater percentage of fine alumina material, e.g. minus 44.mu. particle size material, than the starting or initial alumina dust material itself.

Accordingly, it is an object of this invention to provide a process for the coarsening or agglomeration of very fine particle size alumina or alumina dust.

It is another object of this invention to provide a method for the treatment of partially calcined alumina dust, such as may be derived from the calcining of alumina trihydrate in the manufacture of alumina involving the processing of Bayer pregnant liquor, the precipitation of alumina trihydrate therefrom and the calcining of the resulting precipitated alumina trihydrate.

Still another object of this invention is to provide a method for the agglomeration or coarsening of partially calcined alumina dust recovered from the electrostatic precipitators, mechanical filters or cyclone dust collectors usually employed in the production of alumina from alumina trihydrate produced via the Bayer process.

Yet another object of this invention is to provide an overall process for the production of alumina trihydrate involving the simultaneous or concomitant coarsening or agglomeration of alumina dust, such as very fine partially calcined alumina dust having a particle size such that at least about 80-90% by weight of the alumina dust has a particle size smaller than 44 microns.

How these and other objects of this invention are achieved will become apparent in the light of the accompanying disclosure made with reference to the accompanying drawings wherein:

FIG. 1 shows a flow scheme in accordance with this invention for the coarsening of finely divided alumina dust; and wherein

FIG. 2 shows a flow scheme incorporating the process of this invention in an overall process involving the production of alumina trihydrate by precipitation of alumina trihydrate from Bayer pregnant liquor.

In at least one embodiment of the practices of this invention at least one of the foregoing objects will be achieved.

In accordance with this invention it has been discovered that alumina dust, particularly the very fine alumina dust such as may be generated and recovered during the calcining of alunina trihydrate to produce alumina, is coarsened or agglomerated by forming an aqueous admixture or slurry comprising the alumina dust and finely divided calcium carbonate or calcite, introducing the resulting aqueous admixture or slurry into Bayer pregnant liquor, with or without the presence of added seed alumina trihydrate, and precipitated alumina trihydrate from the resulting Bayer pregnant liquor. The practices of this invention are particularly applicable to the coarsening or agglomeration of the very fine partially calcined alumina dust recovered from the electrostatic precipitators and cyclone dust separators and other dust separating devices usually associated with a calcining apparatus for the calcining of alumina trihydrate to produce alumina. The practice of this invention is particularly applicable, as indicated hereinabove, to the coarsening of superfine, partially calcined alumina dust recovered from the electrostatic precipitators, such alumina dust having a particle size such that essentially all, i.e. at least about 85-95% by weight, of the alumina dust has a particle size smaller than 44 microns (325 mesh), such as a particle size wherein less than about 10% by weight of the dust has a particle size greater than 44 microns. Optionally, excess fine seed alumina trihydrate may be included with the alumina dust for coarsening.

As indicated hereinabove the alumina dust which is advantageously coarsened or agglomerated in accordance with the practice of this invention has an extremely small particle size. For example, alumina dust recovered by means of cyclone dust separators associated with a rotary kiln for the production of calcined alumina from Bayer process alumina trihydrate might have a particle size such that between about 85-92% by weight is smaller than 44 microns or 325 mesh. Such dust is characterized by a loss on ignition (LOI) of about 12-15% by weight after 1 hour at 1000.degree. C. and would comprise about 20% by weight Al.sub.2 O.sub.3.3H.sub.2 O, about 5% by weight .alpha.-Al.sub.2 O.sub.3 and about 60-65% by weight Al.sub.2 O.sub.3.H.sub.2 O and .gamma.-Al.sub.2 O.sub.3.

The partially calcined electrostatic precipitator dust which is a very fine alumina dust is very advantageously coarsened in accordance with the practices of this invention. Such electrostatic precipitator dust usually has a particle size such that 92-98% by weight is smaller than 44 microns (325 mesh) with a substantial amount, such as in the range 25-45% by weight, having a particle size smaller than 10 microns. The loss on ignition of electrostatic precipitator alumina dust is usually about 15-20% by weight after 1 hour at 1000.degree. C. and the composition of the dust is substantially that described hereinabove in connection with the alumina dust recovered from cyclone separators and, as indicated, comprises a mixture of .alpha.-Al.sub.2 O.sub.3, .gamma.-Al.sub.2 O.sub.3, Al.sub.2 O.sub.3.H.sub.2 O and Al.sub.2 O.sub.3.3H.sub.2 O.

It is interesting to note that the alumina dusts capable of being coarsened in accordance with the practices of this invention, particularly the cyclone separator and electrostatic precipitator alumina dust, contain considerable amounts of amorphous boehmite (Al.sub.2 O.sub.3.H.sub.2 O) which, when incorporated in Bayer spent or Bayer pregnant liquors, will tend to dissolve even at low temperatures of about 160.degree.-180.degree. F. Thus, some 5-10% of the dust would dissolve in spent liquor at about 160.degree. F. in the first 30 minutes of mixing and as much as 15-20% of the electrostatic precipitator dust dissolves in Bayer spent liquor at 180.degree. F., such as when 5 grams of electrostatic precipitator dust is added to 500 ml of Bayer spent liquor.

In the practices of this invention it is preferred to employ for the coarsening of the partially calcined alumina dust a Bayer pregnant liquor of the usual industrial concentration or compositions as would be employed for the production and precipitation of alumina trihydrate, such as a Bayer pregnant liquor containing about 100-130 g/l Al.sub.2 O.sub.3, about 160-200 g/l caustic soda (calculated as Na.sub.2 CO.sub.3) and about 230-260 g/l total titratable soda (TTS).

As indicated hereinabove, in the practices of this invention the finely divided alumina dust is coarsened by forming an aqueous slurry or admixture thereof together with finely divided calcium carbonate or calcite. The resulting aqueous slurry or admixture is then incorporated into or added directly to Bayer pregnant liquor and alumina trihydrate then precipitated from the resulting Bayer pregnant liquor. During the precipitation of the alumina trihydrate from Bayer pregnant liquor coarsening or agglomeration of the thus-incorporated or added alumina dust takes place The finely divided calcium carbonate or calcite employed in the aqueous slurry or admixture with the alumina dust may be chemical reagent grade or commercial grade powdered calcium carbonate.

Reference is now made to the accompanying drawings which illustrate various flow schemes embodying the practices of this invention. FIG. 1 of the drawings illustrates schematically a flow scheme embodying the practices of this invention specifically directed to the agglomeration or coarsening of alumina dust.

As illustrated in FIG. 1 alumina dusts from a suitable source, not shown, are supplied via line 11 to mixing tank 12 equipped with agitator 14 or other equivalent mixing device. There is also introduced into mixing tank 12 via line 15 a slurry of calcium carbonate or calcite in wash water. If desired, as illustrated, excess fine seed alumina trihydrate particles might be introduced into mixing tank 12 with the alumina dust. The resulting aqueous admixture or slurry of alumina dusts, seed alumina trihydrate and calcium carbonate is withdrawn from mixing tank 12 via line 16 and supplied via outlets 16a, 16b and 16c to batch precipitator tanks 18a, 18b and 18c, respectively. Batch precipitator tanks 18a, 18b and 18c are supplied with Bayer pregnant liquor from a suitable source not shown at a suitable temperature, such as about 180.degree. F., via line 19 and outlets 19a, 19b and 19c, respectively, as illustrated.

Within batch precipitator tanks 18a, 18b and 18c precipitation of alumina trihydrate from the Bayer pregnant liquor introduced thereinto occurs over a period of hours, such as about 8-12 hours, more or less, the precipitation taking place with cooling of the Bayer pregnant liquor therein from the charge temperature of about 180.degree. F. to a temperature of about 160.degree. F., the temperature drop within each of the batch precipitators being in the range about 15.degree.-30.degree. F., more or less. As illustrated, provision is made for the circulation of the contents of batch precipitator tanks 18a, 18b and 18c via lines 20a and 21b, 20b and 21b, 20c and 21c, respectively, as illustrated.

During precipitation of the alumina trihydrate from the Bayer pregnant liquor within batch precipitator tanks 18a, 18b and 18 c, coarsening or agglomeration of the alumina dust introduced thereinto occurs. Upon completion of the alumina trihydrate precipitation with resulting coarsening of the alumina dusts within batch precipitator tanks 18a, 18b and 18c the contents of the tanks are drawn off via lines 20a, 22a, lines 20b, 22b and lines 20c, 22c, and through valves 24a, 24b and 24c and line 25, respectively, as illustrated, into pump-off tank 26. From pump-off tank 26 the aqueous slurry wherein comprising coarsened or agglomerated alumina dust, precipitated alumina trihydrate and alumina trihydrate fines and at least partially depleted Bayer pregnant liquor is removed via line 28 and moved by pump 29, transported via line 30, as illustrated, for further handling and processing.

In the operation of the alumina dust coarsening or agglomeration operation illustrated in FIG. 1 and described hereinabove, it is desirable that the amount of alumina dust supplied to the Bayer pregnant liquor within the batch precipitator tanks 18a, 18b and 18c be an amount in the range about 25-150, preferably about 40-60 grams per liter of Bayer pregnant liquor. Along with the alumina dust to be coarsened there is admixed with the Bayer pregnant liquor finely divided calcium carbonate or calcite, such as an amount in the range from about 50-100 to about 200-300, preferably about 80-150 mg, per liter of Bayer pregnant liquor. Also, along with the alumina dust and calcium carbonate added to the Bayer pregnant liquor there is added, usually in the make-up of the aqueous slurry or admixture containing the alumina dust and/or calcium carbonate, water in an amount in the range about 50-150 ml per liter of Bayer pregnant liquor. It is also desirable, as illustrated, that the batch precipitator tanks be operated with cooling, such as from a temperature of about 175.degree.-185.degree. F., e.g. 180.degree. F., the temperature of the Bayer pregnant liquor supplied thereto, to a temperature of about 145.degree.-170.degree. F., e.g. 160.degree. F., the discharge temperature of the partially depleted Bayer pregnant liquor. The batch precipitation operation within each of batch precipitator tanks 18a, 18b and 18c is conveniently carried out over a number of hours, such as from about 5 to about 36 hours, desirably in the range about 8-12 hours, more or less. There would be produced as a result of these operations a slurry comprising agglomerated alumina dust and alumina trihydrate particles wherein the solids content would have a particle size such that at least about 70% by weight of the solids have a particle size greater than 325 mesh in a liquor having an alumina:caustic ratio of about 0.470. If desired, instead of three batch precipitator tanks, only one may be employed or more than one, as illustrated, such as 2 to 5 tanks or more.

Reference is now made to FIG. 2 of the drawings which shows another embodiment of the practices of this invention wherein the alumina dust coarsening or agglomeration operation in accordance with this invention is incorporated in a Bayer process plant for the recovery of alumina, as alumina trihydrate, from alumina ores, such as bauxite. The flow scheme of FIG. 1 is embodied and incorporated in FIG. 2, as indicated by dashed lines 33 and the same reference numerals which were employed to identify and describe the flow scheme of the special embodiment of the practice of this invention illustrated in FIG. 1 have been employed in FIG. 2.

Referring to FIG. 2 of the drawings, alumina ore, such as bauxite, from a suitable source, not shown, is supplied via line 31 to digesters 32 in series wherein the alumina ore is digested with aqueous caustic to yield a saturated or supersaturated solution of sodium aluminate, the Bayer pregnant liquor which is recovered via line 34 and passed through mud separator or filters 35 for the removal of solid impurities, such as iron oxide and the like. The resulting filtered Bayer pregnant liquor is recovered via line 36. A portion, usually a minor portion, about 2-20% by volume, more or less, of the solids-free Bayer pregnant liquor leaving mud separator 35 via line 36, is supplied via valved line 19 for use in the alumina dust agglomeration operation in accordance with this invention as described herein with reference to FIG. 1.

The remaining or major portion of the Bayer pregnant liquor is passed via line 36 to cooler 38 wherein its temperature is dropped, such as from about 180.degree. F. as indicated, to a suitable lower temperature, e.g. to about 160.degree. F. The resulting cooled Bayer pregnant liquor is then transferred via line 39 to precipitators 40a and 40b in series, as illustrated, to effect precipitation of alumina trihydrate from the cooled Bayer pregnant liquor in the presence of added seed alumina trihydrate fines.

Upon precipitation of the alumina trihydrate from the Bayer pregnant liquor within precipitators 40a and 40b the resulting Bayer pregnant liquor, now containing precipitated alumina trihydrate, is recovered via line 41 and admixed with the stream containing agglomerated or coarsened alumina dust alumina trihydrate recovered from the alumina dust coarsening operation via line 30, see FIG. 1. The resulting admixture is then supplied via line 41 to interstage cooler 42 and the resulting cooled Bayer pregnant liquor containing precipitated alumina trihydrate and agglomerated alumina dust is passed via line 44 through precipitators 40c and 40d in series to substantially complete the precipitation of alumina trihydrate from the Bayer pregnant liquor. There issues from precipitator 40d via line 45 a spent Bayer liquor having a substantially reduced sodium aluminate content, usually at a temperature of about 120.degree.-135.degree. F., such as about 125.degree. F. and containing dispersed or suspended therein precipitated alumina trihydrate and the coarsened alumina dust. This Bayer liquor is then supplied via line 45 to settling tanks 46a, 46b and 46c. The larger particle size solids, particularly the larger particle size alumina trihydrate, is usually recovered as product, from settling tank 46a via line 48 for subsequent handling or treatment, such as calcining, for the manufacture of alumina. The finer particle size precipitated alumina trihydrate is usually recovered from settling tank 46b via line 49 and settling tank 46c via line 50 and passed via line 51 and 52 to precipitators 40a, 40b, 40c and 40d as seed alumina trihydrate to promote the precipitation of alumina trihydrate from the Bayer pregnant liquor undergoing treatment and precipitation within precipitators, 40a, 40b, 40c and 40d. Also, as indicated, a portion of the alumina trihydrate fines recovered from settling tank 46c may be transferred via line 50 for admixture with the alumina dust supplied via line 11, or with the calcium carbonate supplied via line 15 to the alumina dust coarsening or agglomeration operation as illustrated in FIG. 2 and more fully described in connection with FIG. 1.

In the practice of the embodiment of the invention illustrated in FIG. 2, it is preferred that the stream containing agglomerated or coarsened alumina dust and precipitated alumina trihydrate supplied via line 30 for admixture with the Bayer liquor flowing in line 41 to interstage cooler 42 to be at a temperature substantially the same as the temperature of the partially spent Bayer liquor leaving precipitator 40b. As illustrated in FIG. 2, the temperature of the stream in line 30 is about 160.degree. F. Accordingly, it would be convenient for the temperature of the partially spent Bayer liquor leaving precipitator 40b be at a temperature of 160.degree. F. However, it should be appreciated that the temperature of the stream in line 30 may be greater than or less than 160.degree. F., such as a temperature in the range 175.degree.-150.degree. F. and the temperature of the stream leaving precipitator 40b via line 41 may be greater or less than 160.degree. F., such as a temperature in the range 175.degree.-150.degree. F., more or less.

Further, as illustrated in FIG. 2, the stream of pregnant Bayer liquor removed from mud separator 35, line 36 and valved line 19 is at a temperature of about 180.degree. F. but may be greater or less, such as at a temperature in the range 195.degree.-170.degree. F. Desirably, the stream of Bayer liquor removed via valved line 19, as illustrated, is removed prior to cooling the Bayer liquor in cooler 38. If desired, however, the liquid stream in valved line 19 may be removed after some cooling of the Bayer liquor has been initiated in cooler 38, such as from line 39, and, if desired, the liquid stream removed via valve line 19 may be removed from the Bayer pregnant liquor after some precipitation of alumina trihydrate has occurred, such as from precipitator 40a and 40b.

For convenience in illustrating the practices of this invention, FIG. 2 shows four precipitators in series. In actual commercial practice the number of precipitators employed may be as many as twelve or more with the sidestream containing the agglomerated alumina being returned to the main flow after about the third, fourth or fifth precipitator in series.

In my above-identified patent application, Ser. No. 182,481, I have described a technique for the coarsening of Bayer alumina trihydrate fine seed in the course of alumina trihydrate precipitation. In accordance with the practices of the invention disclosed in my patent application, Ser. No. 182,481, which involves the incorporation of calcium carbonate in the Bayer pregnant liquor undergoing precipitation for the production of alumina trihydrate, the improvement in coarsening of the precipitated alumina trihydrate is in the area of about 5-15 percentage points plus 44.mu. alumina trihydrate fraction. In the practices of the subject invention, however, the coarsening effect of the water-calcite treatment on alumina dust to form a slurry and the incorporation of the resulting slurry in Bayer process liquor undergoing cooling for precipitation of trihydrate therefrom is dramatic with respect to the agglomeration of the very vine particle size alumina dust. The resulting coarsening and agglomeration is about 25 times greater following the practices of this invention as compared with the absence thereof. On the basis of the plus 44.mu. fraction of the resulting agglomerated alumina dust product, the practices of this invention gave 2 to 3 times better results with respect to cyclone-separated dust and over 25 times better results with respect to electrostatic precipitator alumina dust. These dramatic results are set forth in Tables I, II and III, compare sample 1 with samples 4 and 6 of Table I and samples 3 and 5 of Table III.

In the test results reported in Tables II and III, the electrostatic precipitator alumina dust contained a 6% by weight plus 44.mu. fraction and at alumina dust charges of 50 g/l without water-calcite or calcium carbonate pretreatment or addition there was produced an alumina product having only 3.2% by weight greater than 44.mu.. This product was even finer than the starting electrostatic precipitator alumina dust. When, however, the practices of this invention were employed, such as with the addition of 100 ml of water and 250 mg of finely divided calcium carbonate, the resulting coarsened alumina product obtained 82.4% by weight alumina particle having a particle size greater than 44.mu..

In addition to the excellent coarsening effect, greater productivity was obtained from 62 g of net alumina trihydrate precipitate as compared with 63.2 g of precipitate, compare sample 3 with sample 5 in Table III.

The preferred practice of this invention is to employ a separate batch-type agglomeration-precipitation treatment of the alumina dust and not to mix the alumina dust with the fine Bayer process seed in the production plant for the production of alumina trihydrate. The preferred practice of this invention is illustrated in FIG. 2 of the drawings.

In the preferred practice of this invention involving the coarsening-agglomeration of alumina dust concomitantly with the precipitation of alumina trihydrate from Bayer pregnant liquor, the preferred amount of added calcium carbonate for incorporation along with alumina dust in Bayer process liquor during the coarsening operation is in the range about 100-250 mg of calcium carbonate per liter of Bayer process liquor. The process of this invention for the coarsening-agglomeration of alumina dust provides a very practical solution for the economic reutilization of large quantities of alumina dust in Bayer plants producing smelter-type alumina.

Comparative laboratory agglomeration tests employing the practices of this invention were carried out with different partially calcined alumina dusts. The results of these tests are set forth in accompanying Table IV:

It is seen from Table IV that the calcite-water treatment in accordance with the practices of this invention has considerable effect on the agglomeration of Dust No. 2. In the case of a high ratio starting liquor of 0.670, there was obtained asn agglomerated alumina product of 30.4 plus 44.mu. without treatment and a product of 78 plus 44.mu. with the treatment in accordance with this invention. The same indices for low ratio liquor of 0.620 were 21.2% plus 44.mu. and 58% plus 44.mu., respectively. The agglomeration of Dust No. 2 is acceptable but the agglomeration of Dust No. 1, as indicated in Table IV, is dramatic and exceptional, i.e. from 2.8% plus 44.mu. without treatment to 76.5% plus 44.mu. when treated in accordance with this invention. It would appear that a reason for this is that Dust No. 1 contains at least 3 times the amount of extreme fines of 1-10.mu. than Dust No. 2. Dust No. 2 is more equivalent to cyclone separated fines (Multiclone dust) than electrostatic precipitator dust. Also Dust No. 1 contains only a small fraction of carbon as compared with Dust No. 2.

The results of additional tests showing the advantages of the practices of this invention are set forth in accompanying Tables V, VI and VII.

Table V shows that excess fine seed can be substituted for part of the dust charged and the agglomeration process still works satisfactorily.

Table VII shows that the process is not dependent on the nature of the industrial Bayer liquor and that it also works in synthetic laboratory prepared liquor. It also shows that calcite charges above 250 mg/l do not show additional improvement in effect.

Table VII shows the agglomeration effect over the whole range of particle sizes and not just for the 44 micron size limit. The data of Table VII also confirms that the practice of the invention is applicable to both synthetic and actual commercial Bayer liquors.

The preferred practice of this invention is indicated in FIG. 1 of the drawings. In this embodiment, as indicated in FIG. 2, a secondary stream from the primary Bayer pregnant liquor stream would be withdrawn and employed for the rapid agglomeration of the alumina dusts and excess hydrate fines, such as materials smaller than about 44.mu. (325 mesh). The secondary stream or side-stream removed from the primary Bayer pregnant liquor stream would be in the range about 5-10% vol. of the total Bayer pregnant liquor flow. This secondary stream in accordance with the practices of this invention would then be seeded with a mixture comprising dust and optionally excess hydrate fines along with calcite and water or wash liquor at a temperature about 180.degree.-160.degree. F. and agglomerated in a small batch precipitator, about 3-5 in number, working on very short 8-12 hour cycles, down to drawoff ratios of about 0.470. A continuous pump-off from the resulting agglomeration would than be sent to the main circuit somewhere before the interstage cooler as indicated in FIG. 2. The main advantages of such a side-stream agglomeration operation are,

1. effective fast coarsening of fine partially calcined alumina dust,

2. little need for elaborate precipitation control measures in the main alumina trihydrate precipitation circuit since unusual fluctuations in dust and fines generation would be corrected in the agglomeration side-stream operation in accordance with this invention, and

3. the alumina product produced would be expected to be all plus 44.mu. (325 mesh).

As will be apparent to those skilled in the art in the light of the foregoing disclosure, many modifications, alterations and substitutions are possible in the practice of this invention without departing from the spirit or scope thereof: