AU623821B2 - Method for removal of carbon compounds from circulating liquor of bayer process - Google Patents

Description

11 Shor Int.

AUSTRALIA 3 PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE t Title: Cl: i Ei ii Application Number: Lodged: 'a ,Complete Specification Lodged: Accepted: Lapsed: "Published:

I

Siority: Related Art: Related Art: n

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1i .:i i i'l TO BE COMPLETED BY APPLICANT Name of Applicant: SHOWA DENKO KABUSHIKI KAISHA Address of Applicant: 10-12, Shiba Daimon 2-chome, Minato-ku, Tokyo, Japan.

Actual Inventors: Yasuo Kawai and Yuji Sibue Address for Service: CALLINAN LAWRIE, 278 High Street, Kew, 3101, Victoria, Australia Complete Specification for the invention entitled: "METHOD FOR REMOVAL OF CARBON COMPOUNDS FROM CIRCULATING LIQUOR OF BAYER PROCESS" The following statement is a full description of this invention, including the best method of performing it known to me:- No Legalization No Corporae Sl this 5th day of March 19 91- Signature of Declarant To: The Commissioner of Patents.

la Method for Removal of Carbon Compounds from Circulating Liquor of Bayer Process BACKGROUND OF INVENTION 1. Field of Invention The present invention relates to a method for removal of carbon compounds, such as organic compounds, sodium carbonate, and the like from in the Bayer liquor for producing alumina hydrate or alumina.

2. Description of Related Arts The Bayer process for the manufacture of alumina is a procedure which comprises dissolving an alumina-containing ore, 0 usually bauxite, in conjunction with caustic soda solution in a ooio1 digester at an elevated temperature and high pressure thereby o 0a causing the alumina component of the bauxite to be extracted in the form of sodium aluminate into the liquid; separating .o insoluble components, called "red mud", from the liquid; adding seeds (crystalline alumina hydrate) to the supernatant sodium aluminate; stirring the seeds and supernatant sodium aluminate thereby causing hydrolysis reaction and hence precipitation of crystalline alumina hydrate; and, isolating, washing and drying o" o the alumina hydrate. This is uncalcined alumina hydrate and finds utility as the raw material for various alumina compounds and for activated alumina and as the filler for synthetic resin.

The uncalcined alumina hydrate may be calcined at high temperature to provide an alumina product. The liquid, which remains after the separation of the alumina hydrate, and a part of the washing liquid are concentrated and are circulated for re-use in the Bayer process.

As is evident from the explanation given above, in the system operated for the manufacture of alumina by the Bayer process, the liquids containing the sodium and aluminum components are recycled. In the present invention, all of the liquids are collectively referred to as "circulating liquor of Bayer process". The term "carbon compounds" used in the present invention refers to all of the carbon compounds, such as organic carbon-containing compounds, sodium oxalate, and inorganic carbon-containing compounds, such as sodium carbonate.

-2- Carbon compounds pass into the circulating liquor in Bayer process due to the following major causes.

Organic compounds, such as humin, bitumen, and butin, I which remain unseparated from the starting bauxite ore, are dissolved and accumulated in the digesting step.

A synthetic high-molecular flocculant or a natural flocculant is used for the purpose of accelerating sedimentary separation during the separation of red mud or the separation of i alumina hydrate. The flocculant accumulates in the circulating io liquor of Bayer process.

1 The Bayer liquor foams when it takes in an organic substance. To preclude this foaming, a defoaming agent which is a carbon compound such as a fatty acid ester is used at various i stages throughout the process and is accumulated in the J i.5 circulating liquor in Bayer process.

Carbonate contained in the starting caustic soda accumulates in the circulating liquor of Bayer process. When the circulating liquor of Bayer process is brought into contact with air, carbo-oxidizing occurs thereby forming a sodium carbonate which accumulates in the circulating liquor of Bayer process.

It is known that the presence of carbon compounds in the circulating liquor of Bayer process lowers the productivity of the Bayer process and degrades the products in the following ways: low precipitation rate of alumina hydrate; fine precipitation of alumina hydrate; occurrence of scaling in the apparatuses of the Bayer process, thereby causing operational trouble in the apparatuses; poor sedimentary separation of solid and liquid of the crystalline alumina hydrate; and, blackening and lowering whiteness of the alumina hydrate product (c.f.

Australian Patent No. 523, 504 filed by SHOWA ALUMINUM INDUSTRIES K.K. pages 7 and 8).

Some menhods have been suggested with a view to removing the carbon compounds from the circulating liquor of Bayer process. Examples are a pressurized oxygen method (Japanese Examined Patent Publication No. 45-30,548), an ultraviolet ray irradiation method (Japanese Unexamined Patent Publication No.

49-20,097), cooling method (USP 3,508,884), a magnesium -3- 3re a hydroxide co-precipitating method (Japanese Unexamined Patent Publication No. 51-130,692), a causticizing method (USP No.3,120,996 and USP No. 3,341,286), a sodium oxalate removing method (USP No. 3,649,185, USP No. 3,372,984, Japanese Examined Patent Publication No. 4811,480, Japanese Examined Patent Publication No. 53-398, and Japanese Examined Patent Publication No. 53-400), and a bauxite calcining method (Japanese Unexamined Patent Publication No. 4721,395). Of these methods, the causticizing method and the sodium oxalate removing method have io so far been commercially applied.

°ro The principle of the causticizing method comprises o~o converting the sodium carbonate in the circulating liquor of Q000 0. Bayer process with the slaked lime into caustic soda and calcium 0 carbonate and separating the calcium carbonate and removing it from the liquid. The treatment of the liquid by this method must oo be carried out at a low caustic concentration, which results in poor efficiency in its removal. Moreover, the method is, in practical operation, capable of removing only the sodium carbonate.

2 DIn the case of sodium oxalate removing method, since sodium 0o oxalate is precipitated in the form of fine particles in conjunction with the alumina hydrate in the stage for the precipio 0 Ctation of alumina hydrate, this method, in the stage for the filtration of alumina hydrate, separates sodium oxalate in the form of an aqueous solution from the circulating liquor of Bayer process through such treatments as filtration and washing.

Slaked lime is added to the aqueous oxalte solution to form calcium oxalate which is then finally removed. The precipitation and separation of oxalate is accomplished by suitably adjusting the concentration of the liquid and temperature condition. In addition, this method has a disadvantage in that it inevitably entails addition of water to the system for the purpose of dissolving the sodium oxalate.

The present inventors developed a removal method of carbon compounds, which is superior to the conventional various methods in the light of efficiency, which can avoid pollution of the circulating liquor of Bayer process due to addition of an additive substance for the removal of carbon compounds, to 4 the circulating liquor of Bayer process, and which can remove the carbon compounds irrespective of their kinds (Japanese Examined Patent Publication No. 58-46,450, and Australian Patent No. 523,504). The essence of the method resides in adjusting the circulating liquor of Bayer process and/or the substance serving to concentrate or solidify the liquid so that the molar ratio of the aluminum component to the sodium component in terms of A1 2 0 3 /Na 2 0, therein falls in the range of from 1 to 5, and thereafter subjecting the Bayer liquor to the heat treatment at a temperature of from 500 to 1350 0 C thereby substantially producing sodium aluminate and carbon dioxide gas. The carbon compounds are removed in the form of the carbon dioxide gas. The molar ratio of the aluminum component to the sodium component is desired to be great in the light of thoroughly suppressing the JB remaining amount of the caustic soda. If it is too large, S however, the thermal energy required for the heat treatment increases so much as to render the treatment uneconomical. A preferable molar ratio is 5 or less. A more preferable molar ratio is from 1 to 2.

The heat treatment is carried out at a temperature of from o"o o 500 to 1350 0 C, more preferably from 700 to 1000 0 C. At a temperature between 400 and 500 0 C, the sodium components combined with Sorganic compounds and intermediate carbon compounds are converted only to sodium carbonate so that the desired removal of carbon compounds cannot be expected. At a temperature exceeding 1350 0 C, the product resulting from the heat treatment is fused so as to render the operation extremely difficult and impair economy of energy utilization.

The present inventors carried out, after the development of the above described method, its improvement in the industrial application, particularly improvement of process and plants, as described hereinbelow.

The present inventors payed attention to the following facts and features involved in the above described method.

The circulating liquor of Bayer process is concentrated and solidified to yield solid concentrate. This is subjected to the heat treatment in, for example, a rotary kiln or the like. The carbon compounds contained in the solid concentrate are thermally decomposed to form the carbon dioxide gas, thereby removing the carbon compounds as the carbon dioxide gas. The exhausting combustion gas of a rotary kiln has a temperature of from 500 to 800 0 C. Its sensible heat is utilized for vaporizing, s concentrating, drying, and heating the circulating liquor of Bayer process and/or concentrated liquid.

Hot air, which is generated by a hot-air oven separated from the rotary kiln, may be utilized for vaporizing, concentrating, drying and heating the concentrated liquid. In this case, the effective energy in exhaust gas of a rotary kiln o" is recovered by a method other than the vaporization of the o concentrated liquid. The concentrated liquid is converted into e the form of granules during the vaporizing, concentrating, drying, and heating stages, by using, for example a granulating apparatus, in which the exhaust combustion gas from a rotary o kiln is used as a heat source, disclosed in Japanese Examined Publication No. 63-35,572. The granules may be fed to a rotary kiln so as to subject them to the heat treatment.

For the purpose of facilitating the treatments in a rotary o kiln, the present inventors developed a method, in which the o0 granules are classified by an air-classification apparatus e.g., a cyclone or the like, and coarse granules are subjected to the o~ heat treatment, while the remaining granules are returned and recycled to the granulating stage. The present inventors further investigated this method.

°°When the combustion exhaust gas of a rotary kiln and hot air pass through a granulating apparatus and the like, their temperature lowers to 100 300'C, and they contain a few tens grams of fine dust per Nm 3 of the gas, which cannot be collected by a cyclone. The uncollected dust floating in the exhaust gas and the like cannot be caused to emanate to the ambient atmosphere from the view point of precluding the air pollution.

The uncollected fine dust can be collected by a dry-type dust collector, such as a bag filter or an electric dustcollector, and a wet scrubber. This collection involves the drawbacks described hereinafter.

The dust collected by an electric dust collector is as fine 6 as approximately 5pm. When such collected dust is directly returned to a granulating apparatus, the dust merely passes through the apparatus without granulation, and the particle diameter of the granules, which leave the apparatus, becomes fine. When the granulating apparatus is kept operated while feeding the fine dust thereto, the amount of fine dry dust, which is circulated in the removal process of carbon compounds, increases, with the result that the load applied to an air classification apparatus increases, and, hence pressure drop in pipes connected to this apparatus increases.

oo If the fine granules are fed to a rotary kiln disregarding the drawback described above, dusting occurs in the rotary kiln Ssuch that the combustion becomes incomplete and hence operation '1 o of the rotary kiln is maintained with difficulty, from a o practical point of view. It is necessary for maintaining the S operation of a rotary kiln to decrease the combustion air, and hence air flow in the rotary kiln, so as to suppress the dusting. The combustion oil must be correspondingly decreased.

This in turn leads to decreasing the loading amount of the o concentrated liquid. Since the operation efficiency of the al 0 a rotary kiln and electric power unit is drastically lowered, the method for decreasing the combustion air is not economical. In S order to avoid this drawback, a rotary kiln with a large diameter must be used. In this case, the investment cost becomes enormous.

o0 Now is given consideration of a method in which a hot-air generator is provided for drying of the Bayer concentrated liquid, fine dust leaving a rotary kiln is collected by an electric dust collector, and the exhaust gas from the rotary 3o kiln is treated in a system independent of the one for removal of the carbon compounds. Even in this method, when the fine dust is collected by the electric dust collector and is then fed directly to the granulating apparatus, the distribution of particle diameter of granules shifts to a finer side. In order to increase the particle diameter of the granules, a large amount of dust must be circulated in the system consisting of a granulating apparatus, an air-classification apparatus, and a dry-type dust collector. This makes the system to be extremely I_ I ;I~n -7large scale, and, hence, the method now considered is uneconomical in the light of investment cost.

The fine dust is secondary particles of from 2 to 10pm in diameter, in which the primary particles approximate>- 0.5pm in diameter agglomerate, and in which many pores are f)rmed. Such agglomeration is presumably due to the properties of the primary particles; that is, they are fine, have a surface activity, and are electrically energized on the surface by the electrodes of a dust collector. Although the primary particles agglomerate, they are in point-contact with each other and are therefore C. weakly bonded. The fine dust is therefore likely to disintegrate 0000 0o while floating in the air stream.

000 A method for using a water scrubber is now considered.

Q0 3 4 A water scrubber can be used to collect the fine dust which 0000 oo 0 cannot be collected by a cyclone. The waste gas containing fine Go 0 o6 dust is subjected to washing, and the washed water is circulated to the Bayer process. The so-treated waste gas is emitted in air as effluent gas.

Now the chemical composition of the fine dust is considered. The Al 2 0 3 /Na 2 0 ratio of the fine dust is lower than at the Sa vaporizing and concentrating stages. The dust particles contain therefore Na2 0 in the form of, for example, sodium carbonate and sodium hydroxide, which is unreacted with the Al component. In addition, salt, which is contained in the circulating liquor of Bayer process, is also contained in the particles. Because of a the Na components contained in the particles, they are highly reactive. Precipitation of fine crystals easily occurs in waste gas upon temperature fall. The dust particles absorb water contained in the waste gas and are very deliquescent.

It is now considered, what occurs in a water scrubber when the dust particles, which are highly reactive, fine and deliquescent, as described above, are treated in a water scrubber so as to collect the fine dust. Then, scale mainly composed of dausonite (NaAlCO 3 (0H) 2 precipitates on the exhaust-gas duct or the like, with the result that pressure loss increases in the scrubber system. Inevitably, the treating capacity of the scrubber then lowers gradually as in the above described case of using an electric dust-collector.

A

8 If precipitation of the scale is allowed to continue, not only the treating efficiency gradually lowers, but also operation of the wet scrubber becomes difficult and must be stopped mainly for periodic removal of the scale.

In addition, a device for scale removal must be specially provided, and, therefore, the investment cost of the wet scrubber increases and the operating efficiency decreases.

SUMMARY OF INVENTION It is therefore an object of the present invention to provide a method which enables desired removal of carbon compounds to be accomplished with efficiency at least comparable to the conventional methods thereby enhancing the efficiency of the Bayer process and which can remove fine dust not collectible by o the conventional methods thereby preventing the environmental oiz pollution. In the collection of the fine dust, detrimental e influences upon the respective stages for granulating, heatrecovering, dust collecting by a cyclone or the like, which are entailed in the conventional methods for removing the carbon compounds, should be avoided.

o 0It is another object of the present invention to provide a S method for collecting the fine dust as valuable components, and not causing environmental pollution.

In accordance with the present invention, there is provided a method which, in the production of alumina by the Bayer process, effects the removal of carbon compounds contained in the Bayer liquor by: adjusting the circulating liquor of Bayer process or its concentrate so that the molar ratio of the aluminum component to the sodium component in terms of A1 2 0 3 /Na 2 0 therein falls in the range of from 1 to 5; converting the so-adjusted liquid and/or its concentrate to granules; classifying the granules by air classification; subjecting coarse granules to heat treatment at a temperature of from 500 to 1350 0 C; returning the remaining granules to the granulating stage thereby further granulating the same; subjecting the gas from the air classification stage to treatment by a dry-type dust collector thereby collecting the fine dust; rolling and then compressing the fine dust; and, subsequently returning the L. 11 9 fine dust to a heat treating stage.

The air classification is employed in the present invention so as to effectively and easily heat the dust in a heat treating apparatus, a rotary kiln. The size and amount of s granules, which are either heat treated in a rotary kiln or the like or returned to the granulating stage, are adequately determined considering the balance of the total system.

The present invention is further described with reference to the drawings.

BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a flow chart illustrating an embodiment of the S carbon compounds removal method according to the present io invention.

Fig. 2 illustrates a method for granulating and O f dust-collecting according to embodiment of the present o invention.

Figs. 3 and 4 illustrate methods for granulating and dust-collecting according to prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 200 In the present invention, the Bayer liquor may be subjected 0 o to the removal of the carbon compounds at any specific step of the whole Bayer process. It is preferable in the light of 00 00 0 treating efficiency to select the origin at which the concentration of organic compounds, sodium carbonate, sodium oxalate, and the like is high.

An example of a preferable origin is disclosed in Japanese Examined Patent Publication No. 63-6,490 and Japanese Examined Patent Publication No. 1-14,173 proposed by the present inventors, that is, an origin where the concentration of sodium oxalate is highest.

Undissolved residue at the stage for adjusting the molar ratio shown in Fig. 1 is added, if necessary, to the Bayer liquor at any step of the Bayer process, and, subsequently, adjusting of the molar ratio is carried out. The molar ratio is adjusted so that the molar ratio of the aluminum component to the sodium component of the concentrate falls in a range of from 1 to 5 in terms of Al 20 /Na20.

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10 When the Bayer concentrate is treated in accordance with the present invention, the molar ratio is related to the components of the concentrate. The adjusting of the molar ratio is carried out before the concentrating stage in Fig. 1. The molar ratio can, however, be adjusted after the concentrating stage. This concentration is preferably carried out in a multiple-effect evaporator. Details of the molar ratio adjustment mentioned above are disclosed in Australian Patent No. 523,504 (pages 14 18). The descriptions in said patent are referred to herein.

The concentidted liquid is transferred to the granulating o stage. The description hereinafter is made with regard to an embodiment of the concentrated liquid.

The air classification is carried out by two dry-type cyclones 1 and 2. A portion of the dust is circulated in cyclone a 25 1 and the granulating stage. The fine dust, which is collected o. by cyclone 1, is transferred to the granulating stage. The granules, which have been heated, are charged in a granulating apparatus. The concentrated liquid is sprayed onto the granules, so as to convert the liquids to granules.

An embodiment of the granulating apparatus is illustrated A'l' in Fig. 2. This device 4 is a stirrer, for example, a pag mill which comprises a rotary shaft which is rotated by a motor M and is provided with paddles. The concentrated liquid is sprayed into the granulating apparatus from several inlets. The specific method for heating the granulating apparatus 4 is not shown in the drawing. The heating of the granulating apparatus may be carried out so as to heat the granules in the apparatus to an adequate temperature. The heating may not be carried out if Ugranules have been heated in the cycling process prior to charging in the granulating apparatus. In addition, the heating method may be direct or indirect heating. The concentrated liquid is sprayed and applied on the heated granules. At this moment, water of the concentrated liquid vaporises, while leaving and depositing the components other than water on the heated granules. The heated granules act therefore as seeds for the growth of larger granules. Therefore, the larger the

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11 granules charged in the granulating apparatus 4 are, the larger becomes the size of the granules produced by the granulating apparatus 4.

The granules, which are withdrawn out of the granulating apparatus 4, are subjected to air classification. The exhaust gas from the rotary kiln 7, in which oil is burnt with air, is fed by a fan to cyclones 1 and 2. This exhaust gas is used for feeding the granules from the granulating apparatus 4 to cyclone 1. Heat exchange therefore occurs between the granules and the exhaust gas. The granules are therefore heated to a high 00' temperature. Gas for the air classification is not limited to Sthe exhaust gas from the rotary kiln 7 but may be any gas having S a high temperature.

The large granules which have been feeded by the exhaust S1, gas to cyclone i, are collected by this cyclone. A portion of S the collected granules is feeded to a rotary kiln for the heat treatment, while the other portion is feeded to the granulating apparatus 4. Uncollected fine granules, which cannot be collected by cyclone 1I, are collected by cyclone 2 and are a o5 conveyed to the granulating apparatus 4, as a whole.

The granules, which are conveyed to the rotary kiln 7, are thermally heated at a temperature of from 500 to 1350 0 C, preo ferably from 900 to 1100°C. The carbon compounds are decomposed to yield carbon dioxide gas which is emitted from the rotary kiln 7. The carbon compounds are therefore removed from the o granules. Since the rotary kiln 7 is usually operated at a constant temperature. The amount of combustion oil and air is therefore determined in proportion to the amount of solid materials, the granules collected by the cyclone 1. The capacity of the rotary kiln is therefore determined by the amount of combustion oil and air.

When the granules are decarbonized and heated, they are dissolved by water or the circulating liquor of Bayer process.

The liquid is circulated in the Bayer process. The residue, which is undissolved, is used for adjusting the molar ratio of the concentrated liquid and recovering the sodium and aluminum components contained therein.

12 The exhaust gas from cyclone 2 includes fine dusts containing valuable materials. This exhaust gas is therefore treated by an electric dust collector 3 or a dry-type dust collector, such as a bag filter. The exhaust gas from the s electric dust collector 3 is emitted through a fan 6 to ambient air. The fine dust collected by the electric dust collector is an agglomeration of the primary particles weakly bonded with each other as described above. In the present invention, the fine dust is made coarser and is converted to agglomerates io having a higher bonding strength. Then, the fine dust can be returned to the granluating apparatus, with the result that the the ability for removal of carbon compounds is drastically enhanced. The fine dust is therefore subjected to rolling and compressing. Although the present invention is not limited to i. the embodiment shown in the drawing, a screw conveyor 8 is S preferable for the rolling and compressing, because the dust from the electric dust collector can be continuously fed to the screw conveyor 8. During the treatment by the screw conveyor 8, the fine dust is rolled and compressed by the screw, with the 0 result that: the porosity of particles decreases; the particles oare bonded with each other by an appropriate amount of moisture contained therein; and, further, the particles are spheroidized o to granules having a diameter ranging from approximately 20m to Such granules can be directly fed to the rotary kiln 7 or can be fed to the air classification and concentrating stages.

j Such granules are, however, preferably returned to the granulating stages because it is advantageous to further increase the particle diameter.

The present invention is hereinafter described by way of an example and comparative examples.

Example 1 The method according to the present invention was carried out by an experimental plant, whose flow chart is shown in Fig.

2.

13 The circulating liquor of Bayer process was subjected to the adjusting of A I203/Na2 0 ratio to provide the ratio of 1.2 and was concentrated to a specific gravity of 1.70. The resultant concentrated liquid was fed to the granulating apparatus 4. The granules were cycled in the pass connecting cyclones i, 2 and the granulating apparatus 4. The concentrated liquid and granules are therefore fed together into the granulating apparatus 4. The granules are caused to grow in the granulating apparatus and are then conveyed to cyclone 1 with the aid of the exhaust gas from the rotary kiln 7 to cyclone 1. Cyclone 1 separates large granules from the granules fed thereinto. A portion of the separated granules is fed to the rotary kiln 7 and the other portion to the granulating apparatus 4, respectively. Said other portion is therefore fed to the granulating apparatus 4, together with the granules separated in the cyclone 2. Dust, which is uncollected by the cyclone 2, is collected by the electric dust collector and is then rolled and compressed by a screw conveyor 8.

Example 2 (Comparative Example) -201 The method according to the present example is illustrated U'o in Fig. 3, in which the treatments are the same as in Fig. 2, except that the dusts unseparated by cyclone 2 is collected by the water ejector 3'and, further, the screw conveyor is not used.

Example 3 (Comparative Example) The method according to the present example is illustrated in Fig. 4, in which the treatments are the same as in Fig. 2, except that the screw conveyor is replaced with the rotary valve The dust collected by the electric dust collector is therefore returned to the granulating apparatus 4.

The cyclones, granulating apparatus, fan, and rotary kiln in Examples 1 through 3, respectively, had the same size and capacity. The difference resides in therefore the methods for treating the fine dust accompanied with the exhaust gas.

In Example 2, scale accumulated in the water ejector 3' in accordance with continuation of operation, and clogging occured in the water ejector Pressure drop then increased to 14 decrease the gas flow in the system, such that the treating capacity decreased almost a half after the 20-day operation. The operation was therefore stopped to clean the system.

In Example 3, since the fine dust, which was collected by the electric dust collector, was returned to the granulating apparatus, without further treatment, particle diameter of the granules cycling in the system as well as the particle diameter of granules fed to the rotary kiln became finer as the operation proceeds. Therefore, dusting occurred in the rotary kiln, and, in order to avoid this, gas flow had to be decreased to such a level that the treatment amount decreased considerably.

In Example 1, neither clogging nor dusting occurred during the operation, and satisfactory operation with high efficiency was possible for a prolonged period of time.

The operation results of the respective examples are shown in Table 1. In Table i, Process A indicates the outlet of the rotary kiln, where the granules have been heat treated. Process B indicates the outlet of a screw conveyor in Example 1, the inlet of the water ejector in Example 2, and inlet of the rotary valve in Example 3 respectively. The numerals in Table 1 Sare the weight and average particle-diameter Table 1 Example 1 Example 2 Example 3 Process A Solid amount (ton/hr) 3.0 2.5 1.6 Average particle 0.8 0.9 0.4 diameter (cm) Process B Solid amount (ton/hr) 0.4 0.2 1.3 Average particle 63 8 diameter (Pm) Process C Removal amount(ton/hr) 0.11 0.08 0.06 Remarks. Average value of 20-day operation is shown in Example 2.

15 From the comparison of Example 1 with Example 2 it is apparent that the latter treatment amount is lower than the former by approximately 17%. This is because clogging occurred in the wet scrubber, thereby incurring decrease of the air flow in the whole system. From the comparison of Example 1 with Example 3, the latter treatment amount is lower thant the former by 47%. This is because, since the dust returned from the electric dust collector to the granulating apparatus is fine um; cf 6 3 1 the size of granules returned to the rotary kiln becomes fine (0.4cm; cf0.8cm). This makes it necessary to decrease the gas flow so as to avoid the dusting.

t As is apparent from the above examples, the present invention is advantageous over the prior art in the following Spoints.

l The treatment amount of the dust is large, and the operation is stable.

Dusting therefore does not occur due to the other portion of dust returned to the rotary kiln 7, because the particle diameter of such dust in increased by rolling and compresoo-,b sion.

Amount of fine dust circulated in the sytem is low, with the result the load applied for granulating apparatus is Slow.

Particle diameter of dust circulated in the system is approximately ten or more times larger than that of Example 3.

SThis attributes to the advantage of the persent invention over the prior art.

Claims (7)

1. A method for the removal of carbon compounds from Bayer liquor having aluminum compounds and sodium compounds as the principal components, also containing carbon compounds, and circulating through a system for the manufacture of alumina from an alumina-containing ore by the Bayer process, which method comprises the stages of: adjusting the recycling liquid of Bayer process or a concentrate liquid of the Bayer liquor so that the molar ratio of the aluminum component to the sodium component in terms of Al 2 0 3 /Na20 therein falls in the range of from 1 to converting the so-adjusted liquid to granules; classifying the S granules by air classification; subjecting coarse granules S separated by the air classi'ication to heat treatment at a S temperature of from 500 to 1350'C; returning the remaining Sgranules to the granulating stage thereby further granulating the same; subjecting the gas from the air classification stage to treatment by a dry-type dust collector thereby collecting fine dust; rolling and then compressing the fine dust; and, subsequently returning said fine dust to any one of the above- S mentioned stages.

2. A method for the removal of carbon compounds according to claim 1, wherein an electric dust collector is used as said o dry-type dust collector, and, said fine dust is rolled and compressed by a screw conveyor.

3. A method for the removal of carbon compounds according to claim 1 or 2, wherein said heat-treating stage is carried out by a rotary kiln, and, further, the granules, which are withdrawn from the granulating apparatus, are heated by the exhaust gas from the rotary kiln.

4. A method for the removal of carbon compounds according to any one of claims 1 through 3, wherein one cyclone for dry classification and one cyclone for collecting dust is used in said air classification stage. A method for the removal of carbon compounds according to claim 3 or 4, wherein a portion of granules collected by said cyclone for dry classification is fed to said rotary kiln, and the other portion is fed to the granulating apparatus together with dust collected by said cyclone for collecting dust.

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6. A method for the removal of carbon compounds according to any one of claims 1 through 5, wherein said rolled and compressed fine dust is returned to the granulating stage.

7. A method for the removal of carbon compounds from Bayor liquor substantially as hereinbefore described with reference to the accompanying drawings. DATED this Day of February, SHOWA DENKO KABUSHIKI KAISHA By their Patent Attorneys: CALLINAN LAWRIE

1991. -0 'o 0~