AU2007356401B2

AU2007356401B2 - A method for producing alumina with middle and low grade bauxite

Info

Publication number: AU2007356401B2
Application number: AU2007356401A
Authority: AU
Inventors: Songqing Gu; Xinhua Li; Xiangmin Liu; Guobao Wu; Zhonglin Yin
Original assignee: Aluminum Corp of China Ltd
Current assignee: Aluminum Corp of China Ltd
Priority date: 2007-07-12
Filing date: 2007-12-21
Publication date: 2013-10-31
Anticipated expiration: 2027-12-21
Also published as: RU2010104624A; RU2478574C2; CN101113001A; CN100532262C; WO2009006772A1; AU2007356401A1

Abstract

A method for producing alumina with middle and low grade bauxite is characterized in that the Bayer process digestion is performed by adding the dealuminizing residue as an additive instead of lime into middle and low grade bauxite during the Bayer process; the red mud after the Bayer process digestion is subjected to reversal washing, then mixed with recycling alkali liquor, digested after the lime being added, and the resulting red mud is discharged after it is separated, a portion of the resulting solution is used to reversal wash the red mud obtained from Bayer process digestion, then supplemented with alkali after it is sent to Bayer process system, the dealuminizing residue and the recycling alkali liquor for wet-treating the red mud obtained from Bayer process are obtained after lime is added into another portion of the resulting solution, and the dealuminizing residue , i.e. calcium aluminate hydrate, as a additive, is added to the Bayer process digestion instead of lime. The method is a wet-treating process, is suitable for middle and low grade bauxite, and has advantages of simpleness, low energy consumption, low alkali consumption, high yield of alumina, high level of resources utilization, low red mud discharging, and comprehensive utilization of red mud.

Description

H:\gw\Intenovcn\NRPortbl\DCC\GW\5597482 1.doc4/10/2013 Method for Preparing Alumina from a Medium/Low Grade Bauxite Technical Field This invention relates to a method for preparing alumina from a medium/low grade bauxite, especially to a method for preparing alumina by processing a medium/low grade bauxite using a Bayer process. Background Art Currently, there are several industrially available methods for processing a medium/low grade bauxite (A/S<8), such as a sintering process, a series-parallel process, an ore dressing Bayer process, a lime Bayer process, a series process and the like. Among others, the sintering process and the series-parallel process each suffer from a high energy consumption and a complex procedure; the ore dressing Bayer process and the lime Bayer process each have advantages such as a simple procedure and a lower energy consumption, but suffer from a low recovery of alumina (only about 70%) and a poor utilization of the sources. The series process is carried out by first processing bauxite by Bayer process and then processing the Bayer red mud by sintering, as leads to a discharged red mud having a low A/S and a low N/S and a high utilization of resources. Thus, the series process is a good option to processing a medium/low grade bauxite. Owing to the existence of sintering, however, the series process is still relatively complex and has a relatively high energy consumption. Bayer hydration process was proposed in Soviet Union and the like, including processing bauxite and processing red mud as shown in the flow chart of Fig. 1, see Technology for Preparing Alumina, edited by Yang Chongyu, Metallurgical Industry Publisher, Beijing, 1982, p 3 0 7 . Though it is a purely wet process, Bayer hydration process has the following shortcomings: 1) in order to obtain an alkaline solution having a high ak for processing the red mud, it is necessary to deeply evaporate a sodium aluminate solution to have a Na 2 0 concentration of equal to or more than 500g/L so as to precipitate sodium aluminate crystals, as leads to a too huge evaporation amount and a high energy consumption while the sodium aluminate precipitated is difficult to separate; 2) the red mud is processed under stringent conditions such as a high temperature of 280-300'C and H:\gw\lnIerwoven\NRPortbl\DCC\GW\5597482_ Ldoc-4/10/2013 -2 a high alkali concentration of 400-500g/L calculated as Na 2 O, as is difficult to realize in industrial scale; 3) the process flow is complex. Thus, this Bayer hydration process is difficult to put into practice in industrial scale. This invention aims to overcome these shortcomings mentioned above, by providing a totally wet method for preparing alumina by processing a low grade bauxite. This method can have advantages in that the operation temperature is lower, the alkali concentration is lower and the resulting residue is less. Thus, this method can decrease energy consumption significantly and be easily conducted in industrial scale. Summary of the Invention This invention seeks to provide a method for preparing alumina from a medium/low grade bauxite, which is operated in a totally wet mode, has a simple process flow, a low energy consumption, a low alkali consumption, a high recovery of alumina, a high utilization of resources and a low amount of red mud discharged, and is easy to conduct comprehensive utilization, so as to possibly overcome the shortcomings of the prior art. The method can be realized by the following technical solutions. Described herein is a method for preparing alumina from a medium/low grade bauxite, wherein the bauxite is firstly processed by Bayer process and then the red mud obtained is processed in a wet mode by use of an alkaline solution having a low concentration. Accordingly, the present invention provides a method for preparing alumina from a medium/low grade bauxite, characterized in that it includes the following steps: a) a recycling mother liquor and, as digestion additive, a de-aluminized residue are added into the bauxite to conduct Bayer digestion to obtain a Bayer digested slurry, which is separated to obtain a sodium aluminate solution and a Bayer red mud; the sodium aluminate solution is further processed to obtain a precipitated mother liquor and, as product, alumina; and b) in the Bayer red mud from step a), lime and a recycling alkaline solution are added to conduct a Bayer red mud wet processing to give a wet processed slurry, which is separated to obtain a red mud digested liquor and a discharged red mud; H:\gw\lnteroven\NRPortb\DCC\GW\5597482_1.doc-4/10/2013 -3 said discharged red mud is further washed to obtain a discharged red mud wash liquor and an ultimate discharged red mud. Further described is a method for preparing alumina from a medium/low grade bauxite, characterized in that: in the Bayer process used, the digestion step is Bayer digestion during which de-aluminized residue is added instead of lime into the medium/low grade bauxite; the red mud obtained from the digestion is back washed and then combined with a recycling alkaline solution and lime to carry out a further digestion; after the further digestion, the slurry obtained is separated to give a red mud, which is discharged, and a solution, of which a portion is used to back wash the Bayer digested red mud and then fed to the Bayer system to supplement alkali while another portion is reacted with lime newly added to give a de-aluminized residue and a recycling alkaline solution; the recycling alkaline solution is used to take part in the Bayer red mud wet processing, while the de-aluminized residue (i.e., calcium aluminate hydrate) is added into the Bayer digestion step instead of lime. The method can have a suitable process flow and can be easy to operate. For ally currently available series-parallel alumina preparing plant, the incorporation of only one set of wet digestion system can eliminate the need for the sintering system having a high energy consumption and a complex process flow. Thus, the energy consumption needed to prepare alumina can be significantly reduced. The method is characterized in that the discharged red mud contains less amounts of A1 2 0 3 and Na 2 0, and that lime is added in a smaller amount. Thus, the utilization of the resources can be improved and the discharged red mud reduced. The method is characterized in that the discharged red mud has a lower amount of Na 2 0 and thus can facilitate the comprehensive use of red mud. The inventive method is suitable for use in a plant which prepares alumina from a medium/low grade bauxite. The inventive method can have positive effects on improving the utilization of bauxite resource and reducing the energy consumption and alkali consumption in alumina preparation, and thus can have a good perspective. Brief Description of the Drawings Fig. 1 is the flow chart of the Bayer hydration process of the prior art; H:\gw\Intenoven\NRPonbl\DCC\GW\5597482 I.d oc -4/10/201 3 -3A Fig.2 is the flow chart of a totally wet method according to an embodiment of this invention; and Fig.3 is the flow chart of another totally wet method according to a further embodiment of this invention. Best Modes to Carry out the Invention In the following, the present invention will be further described by specific embodiments in combination with the figures. As used in the context, "de-aluminized residue" refers to a solid, which is obtained from the reaction of lime with a solution obtained from the Bayer red mud wet processing, and has a composition mainly consisting of tricalcium aluminate hydrate and a minor amount of silica. As used in the context, "aL" refers to the molar ratio of sodium hydroxide and alumina in a solution. As used in the context, "discharged red mud" refers to the red mud ultimately discharged after bauxite is processed. As used in the context, "recycling alkaline solution" refers to an alkaline solution used in the wet processing of Bayer red mud. According to one aspect of this invention, provided is a method for preparing alumina by processing a medium/low grade bauxite. This method includes two main processes, i.e., Bayer digestion of bauxite and wet processing of the red mud obtained from the Bayer digestion. According to another embodiment of this invention, a medium/low grade bauxite, a recycling mother liquor and a de-aluminized residue, as additive for digestion, are added during the Bayer digestion of bauxite. Preferably, during the Bayer digestion, added are a medium/low grade bauxite having an alumina/silica (A/S) ratio of 3-9, a recycling mother liquor having a Nk of 180-280g/L and a de-aluminized residue, wherein the bauxite and the recycling mother liquor are added in such an amount that the digested liquor obtained has an ak value of 1.35-1.60, and the de-aluminized residue is added in such an amount that CaO thereof is 3-15% of the bauxite added. If the de-aluminized residue is not enough, then CaO can be added to act as part of digestion additives.

H:\gw\Intevoven\NRPottbl\DCC\GW\5597482_I.doc-4/10/2013 - 3B According to another embodiment of this invention, the Bayer digestion can be realized by a Bushing Preheating-Retention Tank digestion technique, a Tube digestion technique, a Bushing Preheating-Autoclave digestion technique, a double stream digestion technique and the like, which are well know in the art and thus will not be further explained in the following. Preferably, the digestion is conducted at a temperature of 240 290'C for 10-120 minutes. According to another embodiment of this invention, the Bayer digested slurry is further diluted, sedimented and separated to obtain a sodium aluminate solution. The sodium aluminate solution is further seed precipitated to obtain a precipitated mother liquor and aluminum hydroxide. The aluminum hydroxide is calcinated to obtain the resultant alumina. The precipitated mother liquor is evaporated to give a recycling mother liquor, which is fed into the Bayer digestion step. The red mud obtained from the sedimentation and separation of the slurry is washed by hot water, giving a Bayer red mud having an alumina/silica ratio (A/S) of < 1.7 and a sodium oxide/silica ratio (N/S) of 0.20 0.65. As shown in Fig.2, optionally, a portion of the red mud digested liquor obtained from the wet processing of Bayer red mud (as shown later) is used to back wash the Bayer digested red mud. Said back washing can be done in a well-known red mud filtering-washing mode or in a settling tank washing mode. Redundant back wash liquor partly is de-aluminized and partly is fed into the precipitated mother liquor. 5 Alternatively, as shown in Fig.3, optionally, the discharged red mud wash liquor (as shown in the following) is used to back wash the Bayer digested red mud. Redundant back wash liquor is de-aluminized. The Bayer digested red mud (alternatively, after being back washed) is fed into the Bayer red mud wet processing step. That is to say, the Bayer digested red mud 10 (alternatively, after being back washed) is mixed with a recycling alkaline solution having a medium/low concentration and lime. The Bayer digested red mud is added in an amount of 10-350g per L of the recycling alkaline solution. The recycling alkaline solution has a caustic alkali concentration (N) of 30-260g/L, preferably 90-220g/L, more preferably 100-190g/L, and an alumina concentraion of equal to or less than 15 20g/L. The lime is added in such an amount that the C/S ratio (i.e., the mass ratio of CaO to SiO 2 in the discharged red mud) is 0.5-2.5, preferably 1.0-2. 1. The wet processing step is conducted at a temperature of 200-300'C, preferably 200-280'C, more preferably 240-270'C for 5-150 minutes. Said wet precessing can be realized by a Bushing Preheating-Retention Tank digestion technique, a Tube digestion technique, 20 a Bushing Preheating-Autoclave digestion technique, a double stream digestion technique and the like. After the wet processing step of Bayer red mud, the resultant slurry is separated to obtain a red mud digested liquor and a discharged red mud. Lime is added into all or part of the digested liquor to remove alumina. Part of the digested liquor is used to 25 back wash the Bayer red mud, as shown in Fig.2. Alternatively, lime is added into all or part of the digested liquor to remove alumina. Part of the digested liquor is directly fed into the precipitated mother liquor, as shown in Fig.3. The discharged red mud is washed by hot water to obtain an ultimate discharged red mud and a discharged red mud wash liquor. The discharged red mud wash liquor may be used to back wash the 30 Bayer red mud, as shown in Fig. 3, or may be used to separate the slurry obtained from the wet processing of Bayer red mud, as shown in Fig.2, or may be used to separate the slurry obtained from Bayer digestion. The ultimate discharged red mud has an A/S ratio of 0.2-0.8 in general, preferably 0.4-0.8, and a N/S ratio of 0.01-0.30 in general. 35 During the de-aluminization of part of the red mud digested liquor and/or part of the back wash liquor, lime is added in such an amount that the C/A ratio (i.e., the mass ratio of CaO in lime to A1 2 0 3 in the liquor) is 1.5-2.5. The de-aluminization is conducted at a temperature of 40-110O*C for 10-180 minutes. Optionally, caustic alkali is added into the recycling alkaline liquor after de-aluminization, so as to process the 40 next batch of Bayer digested red mud. The resultant de-aluminized residue obtained from the de-aluminization is used as additive for Bayer digestion. The slurry obtained 4 H:\gwv\1nteroven\NRPortbl\DCC\GW\5597482_ 1.doc-4/10/2013 -5 from the de-aluminization is separated by means such as pressure filtration, sedimentation or the like. Pressure filtration is preferable. The addition of new caustic alkali is conducted in the Bayer red mud wet processing system. According to the embodiments of the invention, the Bayer red mud wet processing is carried out at a low temperature and a low alkaline value, and can make the operation simple, the energy consumption low, and the equipment less eroded. For any currently available Series-Parallel alumina preparing plant, incorporation of only one set of wet digestion system can eliminate the need for a sintering system having a high energy consumption and a complex process flow. Thus, the energy consumption needed to prepare alumina can be significantly reduced. According to the embodiments of the invention, lime is added into a significant portion of the solution obtained from the Bayer red mud wet processing and then reacted with alumina in the solution. Thus, alumina in the solution is substantially removed, as realizes the regeneration of the recycling alkaline solution used in the Bayer red mud wet processing. The resultant de-aluminized residue can serve as additive for Bayer digestion. According to the embodiments of the invention, the discharged red mud contains less contents of A1 2 0 3 and Na 2 0, and lime is added in a smaller amount. Thus, the utilization of the resources can be improved and the discharged red mud reduced, and the comprehensive use of red mud can be facilitated. The method of the invention is suitable to prepare alumina from a medium/low grade bauxite. The inventive method can have positive effects on improving the utilization of bauxite resources and reducing the energy consumption and alkali consumption in alumina preparation, and thus can have a good perspective. Next, this invention will be further illustrated by the following specific examples. The skilled in the art shall recognize that this invention is not limited to these examples. Examples Example I A bauxite having an alumina/silica (A/S) ratio of 3 and a recycling mother liquor having a caustic alkali concentration Nk of 230g/L are used. A de-aluminized residue as additive is added in such an amount that the CaO content thereof is 11% of the bauxite H:\gw\lnterwovcn\ ortbl\DCC\GW\5597482_Ldoc4/10/2013 - 5A added. The digestion is carried out by a Bushing Preheating-Retention Tank digestion technique at 260'C for 60 minutes. The digested liquor has a ak value of 1.48, and the digested red mud has a A/S ratio of 1.0. After being separated and washed, the Bayer digested red mud is back washed with a discharged red mud wash liquor in a filtering washing manner. After being back washed, the Bayer red mud is wet processed at 260'C by a Bushing Preheating-Retention Tank digestion technique for 90 minutes, wherein a recycling alkaline solution with a concentration of 130g/L is added, the Bayer red mud is added in an amount of 200g per L of the recycling alkaline solution, and lime is added in a total amount such that the CaO / SiO 2 ratio is 1.3. A portion of the digested liquor obtained from the Bayer red mud wet processing is fed to de-aluminization step. The de-aluminization step is conducted at 900C for 30 minutes with the addition of lime in such an amount that the CaO/Al 2 0 3 mass ratio is 1.6. The de-aluminized slurry is separated by pressue filtration with a press-filter. The resultant de-aluminized residue is fed into the Bayer system. The solid phase from the 5 Bayer red mud wet processing, i.e., the discharged red mud, is further washed, and the wash liquor obtained from such further washing is used to back wash the Bayer red mud. The ultimate discharged red mud obtained from such further washing has an A/S ratio of 0.6 and a N/S ratio of 0.10. 10 Example 2 A bauxite having an alumina/silica (A/S) ratio of 5 and a recycling mother liquor having a caustic alkali concentration Nk of 230g/L are used. A de-aluminized residue as additive is added in such an amount that the CaO content thereof is 5% of the bauxite added. The digestion is carred out by a Bushing Preheating-Retention Tank 15 digestion technique at 260*C for 60 minutes. The digested liquor has a ak value of 1.40, and the digested red mud has a A/S ratio of 1. 1. After being separated and washed, the Bayer digested red mud is back washed with a discharged red mud wash liquor in a filtering-washing manner. After being back washed, the Bayer red mud is wet processed at 2600C by a Bushing Preheating-Retention Tank digestion technique 20 for 60 minutes, whererin a recycling alkaline solution with a concentration of 130g/L is added, the Bayer red mud is added in an amount of 250g per L of the recycing alkaline solution, and lime is added in a total amount such that the CaO / Si0 2 ratio is 1.0. All the digested liquor obtained from the Bayer red mud wet processing is fed to the de-aluminization step. The de-aluminization step is conducted at 90'C for 30 25 minutes with the addition of lime in such an amount that the CaO/A 2 0 3 mass ratio is 1.8. The slurry from the de-aluminization step is separated by pressue filtration with a press-filter. The resultant de-aluminized residue is fed into the Bayer system. The ultimate discharged red mud obtained has an A/S ratio of 0.7 and a N/S ratio of 0.10. 30 Example 3 A bauxite having an alumina/silica (A/S) ratio of 7 and a recycling mother liquor having a caustic alkali concentration Nk of 230g/L are used. A de-aluminized residue as additive is added in such an amount that the CaO content thereof is 5% of the bauxite added. Additionally, 2% of lime is added. The digestion is carred out by a 35 Bushing Preheating-Retention Tank digestion technique at 260*C for 60 minutes. The digested liquor has a ak value of 1.50, and the digested red mud has a A/S ratio of 1.2. After being separated and washed, the Bayer digested red mud is back washed with a discharged red mud wash liquor in a filtering-washing manner. After being back washed, the Bayer red mud is wet processed at 260 *C by a Bushing 6 Preheating-Retention Tank digestion technique for 90 minutes, whererin a recycling alkaline solution with a concentration of 170g/L is added, the Bayer red mud is added in an amount of 200g per L of the recycing alkaline solution, and lime is added in a total amount such that the CaO / SiO 2 ratio is 1.7. All the digested liquor obtained 5 from the Bayer red mud wet processing is fed to the de-aluminization step. The de-aluminization step is conducted at 90*C for 30 minutes with the addition of lime in such an amount that the CaO/Al 2 0 3 mass ratio is 1.6. The slurry from the de-aluminization step is separated by pressue filtration with a press-filter. The resultant de-aluminized residue is fed into the Bayer system. The ultimate discharged 10 red mud obtained has an A/S ratio of 0.7 and a N/S ratio of 0.16. Example 4 A bauxite having an alumina/silica (A/S) ratio of 9 and a recycling mother liquor having a caustic alkali concentration Nk of 230g/L are used. A de-aluminized residue 15 as additive is added in such an amount that the CaO content thereof is 9% of the bauxite added. The digestion is carred out by a Bushing Preheating-Retention Tank digestion technique at 2600C for 60 minutes. The digested liquor has a tk value of 1.48, and the digested red mud has a A/S ratio of 1.4. After being separated and washed, the Bayer digested red mud is back washed with a discharged red mud wash 20 liquor in a filtering-washing manner. After being back washed, the Bayer red mud is wet processed at 260*C by a Tube digestion technique for 90 minutes, whererin a recycling alkaline solution with a concentration of 150g/L is added, the Bayer red mud is added in an amount of 1OOg per L of the recycing alkaline solution, and lime is added in a total amount such that the CaO / SiO 2 ratio is 1.9. A portion of the digested 25 liquor obtained from the Bayer red mud wet processing is fed to the de-aluminization step. The de-aluminization step is conducted at 9000 for 30 minutes with the addition of lime in such an amount that the CaO/Al 2 0 3 mass ratio is 1.6. The slurry from the de-aluminization step is separated by pressue filtration with a press-filter. The resultant de-aluminized residue is fed into the Bayer system. The ultimate discharged 30 red mud obtained has an A/S ratio of 0.5 and a N/S ratio of 0.10. Example 5 A bauxite having an alumina/silica (A/S) ratio of 5 and a recycling mother liquor having a caustic alkali concentration Nk of 230g/L are used. A de-aluminized residue 35 as additive is added in such an amount that the CaO content thereof is 13% of the bauxite added. The digestion is carred out by a Bushing Preheating-Retention Tank digestion technique at 260*C for 90 minutes. The digested liquor has a ak value of 1.48, and the digested red mud has a A/S ratio of 1.5. After being separated and washed, the Bayer digested red mud is back washed with a discharged red mud wash 7 liquor in a filtering-washing manner. After being back washed, the Bayer red mud is wet processed at 260'C by a Bushing Preheating-Retention Tank digestion technique for 90 minutes, whererin a recycling alkaline solution with a concentration of 130g/L is added, the Bayer red mud is added in an amount of 50g per L of the recycing 5 alkaline solution, and lime is added in a total amount such that the CaO / SiO 2 ratio is 1.9. All the digested liquor obtained from the Bayer red mud wet processing is fed to the de-aluminization step. The de-aluminization step is conducted at 90'C for 30 minutes with the addition of lime in such an amount that the CaO/Al 2 0 3 mass ratio is 1.6. The slurry from the de-aluminization step is separated by pressue filtration with a 10 press-filter. The resultant de-aluminized residue is fed into the Bayer system. The ultimate discharged red mud obtained has an A/S ratio of 0.7 and a N/S ratio of 0.16. Example 6 A bauxite having an alumina/silica (A/S) ratio of 5 and a recycling mother liquor 15 having a caustic alkali concentration Nk of 230g/L are used. A de-aluminized residue as additive is added in such an amount that the CaO content thereof is 15% of the bauxite added. The digestion is carred out by a Bushing Preheating-Retention Tank digestion technique at 260*C for 90 minutes. The digested liquor has a ak value of 1.48, and the digested red mud has a A/S ratio of 1 .1. After being separated and 20 washed, the Bayer digested red mud is back washed with a discharged red mud wash liquor in a filtering-washing manner. After being back washed, the Bayer red mud is wet processed at 280*C by a Bushing Preheating-Retention Tank digestion technique for 30 minutes, whererin a recycling alkaline solution with a concentration of 130g/L is added, the Bayer red mud is added in an amount of 250g per L of the recycing 25 alkaline solution, and lime is added in a total amount such that the CaO / Si0 2 ratio is 1.3. All the digested liquor obtained from the Bayer red mud wet processing is fed to the de-aluminization step. The de-aluminization step is conducted at 900C for 30 minutes with the addition of lime in such an amount that the CaO/A 2 0 3 mass ratio is 1.7. The slurry from the de-aluminization step is separated by pressue filtration with a 30 press-filter. The resultant de-aluminized residue is fed into the Bayer system. The ultimate discharged red mud obtained has an A/S ratio of 0.65 and a N/S ratio of 0. 10. Example 7 A bauxite having an alumina/silica (A/S) ratio of 5 and a recycling mother liquor 35 having a caustic alkali concentration Nk of 230g/L are used. A de-aluminized residue as additive is added in such an amount that the CaO content thereof is 11% of the bauxite added. The digestion is carred out by a Bushing Preheating-Retention Tank digestion technique at 250*C for 90 minutes. The digested liquor has a ak value of 8 1.48, and the digested red mud has a A/S ratio of 1 .1. After being separated and washed, the Bayer digested red mud is back washed with a discharged red mud wash liquor in a filtering-washing manner. After being back washed, the Bayer red mud is wet processed at 250C by a Tube digestion technique for 90 minutes, whererin a 5 recycling alkaline solution with a concentration of 210g/L is added, the Bayer red mud is added in an amount of 300g per L of the recycing alkaline solution, and lime is added in a total amount such that the CaO / SiO 2 ratio is 1.9. All the digested liquor obtained from the Bayer red mud wet processing is fed to the de-aluminization step. The de-aluminization step is conducted at 90*C for 30 minutes with the addition of 10 lime in such an amount that the CaO/Al 2 0 3 mass ratio is 2.0. The slurry from the de-aluminization step is separated by pressue filtration with a press-filter. The resultant de-aluminized residue is fed into the Bayer system. The ultimate discharged red mud obtained has an A/S ratio of 0.55 and a N/S ratio of 0.16. 15 Example 8 A bauxite having an alumina/silica (A/S) ratio of 5 and a recycling mother liquor having a caustic alkali concentration Nk of 210g/L are used. A de-aluminized residue as additive is added in such an amount that the CaO content thereof is 1 1% of the bauxite added. The digestion is carred out by a Bushing Preheating-Retention Tank 20 digestion technique at 2800C for 30 minutes. The digested liquor has a oXk value of 1.48, and the digested red mud has a A/S ratio of 1.1. After being separated and washed, the Bayer digested red mud is back washed with a discharged red mud wash liquor in a filtering-washing manner. After being back washed, the Bayer red mud is wet processed at 240*C by a Bushing Preheating-Retention Tank digestion technique 25 for 120 minutes, whererin a recycling alkaline solution with a concentration of 130g/L is added, the Bayer red mud is added in an amount of 200g per L of the recycing alkaline solution, and lime is added in a total amount such that the CaO / Si0 2 ratio is 2.3. All the digested liquor obtained from the Bayer red mud wet processing is fed to the de-aluminization step. The de-aluminization step is conducted at 90*C for 30 30 minutes with the addition of lime in such an amount that the CaO/Al 2 0 3 mass ratio is 1.8. The slurry from the de-aluminization step is separated by pressue filtration with a press-filter. The resultant de-aluminized residue is fed into the Bayer system. The ultimate discharged red mud obtained has an A/S ratio of 0.75 and a N/S ratio of 0.10. 35 Example 9 A bauxite having an alumina/silica (A/S) ratio of 3 and a recycling mother liquor having a caustic alkali concentration Nk of 230g/L are used. A de-aluminized residue as additive is added in such an amount that the CaO content thereof is ll% of the bauxite added. The digestion is carred out by a Bushing Preheating-Retention Tank 9 digestion technique at 260*C for 90 minutes. The digested liquor has a ak value of 1.48, and the digested red mud has a A/S ratio of 1.0. After being separated and washed, the Bayer digested red mud is back washed with a discharged red mud wash liquor in a filtering-washing manner. After being back washed, the Bayer red mud is 5 wet processed at 260*C by a Bushing Preheating-Autoclave digestion technique for 90 minutes, whererin a recycling alkaline solution with a concentration of 130g/L is added, the Bayer red mud is added in an amount of 250g per L of the recycing alkaline solution, and lime is added in a total amount such that the CaO / Si0 2 ratio is 1.3. A portion of the digested liquor obtained from the Bayer red mud wet processing 10 is fed to the de-aluminization step. The de-aluminization step is conducted at 900C for 30 minutes with the addition of lime in such an amount that the CaO/A 2 0 3 mass ratio is 1.6. The slurry from the de-aluminization step is separated by pressue filtration with a press-filter. The resultant de-aluminized residue is fed into the Bayer system. The ultimate discharged red mud obtained has an A/S ratio of 0.62 and a N/S ratio of 15 0.14. Example 10 A bauxite having an alumina/silica (A/S) ratio of 5 and a recycling mother liquor having a caustic alkali concentration Nk of 230g/L are used. A de-aluminized residue 20 as additive is added in such an amount that the CaO content thereof is 11% of the bauxite added. The digestion is carred out by a double-stream technique at 2600C for 90 minutes. The digested liquor has a ak value of 1.48, and the digested red mud has a A/S ratio of 1.0. After being separated and washed, the Bayer digested red mud is back washed with a discharged red mud wash liquor in a filtering-washing manner. 25 After being back washed, the Bayer red mud is wet processed at 2600C by a Bushing Preheating-Autoclave digestion technique for 90 minutes, whererin a recycling alkaline solution with a concentration of 130g/L is added, the Bayer red mud is added in an amount of 250g per L of the recycing alkaline solution, and lime is added in a total amount such that the CaO / Si0 2 ratio is 0.9. A portion of the digested liquor 30 obtained from the Bayer red mud wet processing is fed to the de-aluminization step. The de-aluminization step is conducted at 700C for 30 minutes with the addition of lime in such an amount that the CaO/Al 2 0 3 mass ratio is 1.6. The slurry from the de-aluminization step is separated by pressue filtration with a press-filter. The resultant de-aluminized residue is fed into the Bayer system. The ultimate discharged 35 red mud obtained has an A/S ratio of 0.75 and a N/S ratio of 0.10. Example I A bauxite having an alumina/silica (A/S) ratio of 5 and a recycling mother liquor 10 having a caustic alkali concentration Nk of 260g/L are used. A de-aluminized residue as additive is added-in such an amount that the CaO content thereof is 11% of the bauxite added. The digestion is carred out by a Tube digestion technique at 280*C for 30 minutes. The digested liquor has a ak value of 1.35, and the digested red mud has a 5 A/S ratio of 1.5. After being separated and washed, the Bayer digested red mud is back washed with a discharged red mud wash liquor in a filtering-washing manner. After being back washed, the Bayer red mud is wet processed at 2800C by a double stream digestion technique for 30 minutes, whererin a recycling alkaline solution with a concentration of 210g/L is added, the Bayer red mud is added in an amount of 350g 10 per L of the recycing alkaline solution, and lime is added in a total amount such that the CaO / SiO 2 ratio is 2.5. A portion of the digested liquor obtained from the Bayer red mud wet processing is fed to the de-aluminization step. The de-aluminization step is conducted at 40*C for 60 minutes with the addition of lime in such an amount that the CaO/Al 2 0 3 mass ratio is 1.6. The slurry from the de-aluminization step is 15 separated by pressue filtration with a press-filter. The resultant de-aluminized residue is fed into the Bayer system. The ultimate discharged red mud obtained has an A/S ratio of 0.75 and a N/S ratio of 0.04. Example 12 20 The bauxite has an alumina/silica (A/S) ratio of 3. The recycling mother liquor has a caustic alkali concentration Nk of 230g/L and a ak value of 3.0. The Bayer digestion is conducted at 250*C. The de-aluminized residue obtained during the regeneration of the recycling alkaline liquor of the Bayer red mud wet processing is used as additive. The digestion lasts for 90 minutes. The digested liquor has a ak value 25 of 1.48. The digested red mud has an A/S ratio of 1.5. The Bayer red mud wet processing is conducted at 2900C for 20 minutes, whererin the recycling alkaline solution used has a concentration of 30g/L and lime is added in a total amount such that the CaO / Si0 2 ratio is 1.0. The digested red mud has an A/S ratio of 0.5 and a N/S ratio of 0.10. 30 Example 13 The bauxite has an alumina/silica (A/S) ratio of 4. The recycling mother liquor has a caustic alkali concentration Nk of 230g/L and a Ck value of 3.0. The Bayer digestion is conducted at 260"C for 60minutes. The digested liquor has a atk value of 35 1.50. The digested red mud has an A/S ratio of 1.4. The Bayer red mud wet processing is conducted at 2600C for 90 minutes, whererin the recycling alkaline solution used has a concentration of 90g/L and lime is added in a total amount such that the CaO /

II

SiO 2 ratio is 1.2. The digested red mud has an A/S ratio of 0.60 and a N/S ratio of 0.15. Example 14 5 The bauxite has an alumina/silica (A/S) ratio of 5. The recycling mother liquor has a caustic alkali concentration Nk of 230g/L and a ak value of 3.0. The Bayer digestion is conducted at 2700C for 60minutes. The digested liquor has a tk value of 1.50. The digested red mud has an A/S ratio of 1.30. The Bayer red mud wet processing is conducted at 2500C for 80 minutes, whererin the recycling alkaline 10 solution used has a concentration of 130g/L and lime is added in a total amount such that the CaO / Si0 2 ratio is 1.4. The digested red mud has an A/S ratio of 0.65 and a N/S ratio of 0.18. Example 15 15 The bauxite has an alumina/silica (A/S) ratio of 6. The recycling mother liquor has a caustic alkali concentration Nk of 230g/L and a ak value of 3.0. The Bayer digestion is conducted at 2800C for minutes. The digested liquor has a ak value of 1.50. The digested red mud has an A/S ratio of 1.2. The Bayer red mud wet processing is conducted at 2400C for 120 minutes, whererin the recycling alkaline solution used 20 has a concentration of 170g/L and lime is added in a total amount such that the CaO / Si0 2 ratio is 1.9. The digested red mud has an A/S ratio of 0.55 and a N/S ratio of 0.12. Example 16 25 The bauxite has an alumina/silica (A/S) ratio of 7. The recycling mother liquor has a caustic alkali concentration Nk of 230g/L and a ak value of 3.0. The Bayer digestion is conducted at 2600C for 70minutes. The digested liquor has a atk value of 1.50. The digested red mud has an A/S ratio of 1.1. The Bayer red mud wet processing is conducted at 2500C for 150 minutes, whererin the recycling alkaline solution used 30 has a concentration of 190g/L and lime is added in a total amount such that the CaO / Si0 2 ratio is 2.1. The digested red mud has an A/S ratio of 0.65 and a N/S ratio of 0.12. Example 17 35 The bauxite has an alumina/silica (A/S) ratio of 8. The recycling mother liquor has a caustic alkali concentration Nk of 230g/L and a ak value of 3.0. The Bayer digestion is conducted at 2600C for 90minutes. The digested liquor has a tk value of 12 H:\gw\Interwoven\NRPotbl\DCC\GW\5597482_l.doc-4/10/20l3 - 13 1.55. The digested red mud has an A/S ratio of 1.1. The Bayer red mud wet processing is conducted at 280'C for 30 minutes, wherein the recycling alkaline solution used has a concentration of 30g/L and lime is added in a total amount such that the CaO / SiO 2 ratio is 1.9. The digested red mud has an A/S ratio of 0.66 and a N/S ratio of 0.13. Example 18 The bauxite has an alumina/silica (A/S) ratio of 9. The recycling mother liquor has a caustic alkali concentration Nk of 230g/L and a ak value of 3.0. The Bayer digestion is conducted at 270*C for 60 minutes. The digested liquor has a ak value of 1.50. The digested red mud has an A/S ratio of 1.25. The Bayer red mud wet processing is conducted at 260'C for 90 minutes, wherein the recycling alkaline solution used has a concentration of 130g/L and lime is added in a total amount such that the CaO / SiO 2 ratio is 0.9. The digested red mud has an A/S ratio of 0.60 and a N/S ratio of 0.15. This invention has been described in reference to the preferable embodiments thereof. However, a skilled in the art shall understand that this invention can be modified and changed without departing from the scope of the appended claims. Throughout this specification and the claims which follow, unless the context requires otherwise, the word "include", and variations such as "includes" and "including", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims

1. A method for preparing alumina from a medium/low grade bauxite, characterized in that it includes the following steps: a) a recycling mother liquor and, as digestion additive, a de-aluminized residue are added into the bauxite to conduct Bayer digestion to obtain a Bayer digested slurry, which is separated to obtain a sodium aluminate solution and a Bayer red mud; the sodium aluminate solution is further processed to obtain a precipitated mother liquor and, as product, alumina; and b) in the Bayer red mud from step a), lime and a recycling alkaline solution are added to conduct a Bayer red mud wet processing to give a wet processed slurry, which is separated to obtain a red mud digested liquor and a discharged red mud; said discharged red mud is further washed to obtain a discharged red mud wash liquor and an ultimate discharged red mud.

2. A method for preparing alumina from a medium/low grade bauxite according to claim 1, characterized in that: lime is added into a portion of the red mud digested liquor from step b) to conduct de-aluminization to give a de-aluminized slurry, which is separated to obtain a de-aluminized residue and a recycling alkaline liquor; said de-aluminized residue is fed to step a) while said recycling alkaline liquor is fed to step b).

3. A method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: before entering into step b), the Bayer red mud from step a) is back washed with a portion of the red mud digested liquor from step b) to obtain a back washed liquor.

4. A method for preparing alumina from a medium/low grade bauxite according to claim 3, characterized in that: a portion of the back washed liquor is fed to the precipitated mother liquor, and lime is added into another portion of the back washed liquor to conduct de-aluminization to obtain a de-aluminized slurry, which is separated to obtain a de aluminized residue, which is fed to step a), and a recycling alkaline solution, which is fed to step b). H:\gw\ntcrwoven\NRPortbl\DCC\GA\S597482_1.doc-4/10/2013 - 15

5. A method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: before entering into step b), the Bayer red mud from step a) is back washed with the discharge red mud wash liquor from step b) to obtain a back washed liquor.

6. A method for preparing alumina from a medium/low grade bauxite according to claim 5, characterized in that: lime is added into the back washed liquor to conduct de aluminization to obtain a de-aluminized slurry, which is separated to obtain a de aluminized residue, which is fed to step a), and a recycling alkaline solution, which is fed to step b).

7. A method for preparing alumina from a medium/low grade bauxite according to claim 1, characterized in that: before entering into the recycling mother liquor, said precipitated mother liquor is evaporated.

8. A method for preparing alumina from a medium/low grade bauxite according to any of claims 1-7, characterized in that: in step a), the Bayer digestion is conducted at 240 300'C for 10-120 minutes, wherein the bauxite has an A/S ratio of 3-9, the recycling mother liquor has a Nk of 180-280g/L, the bauxite and the recycling mother liquor are added in such an amount that the Bayer digested slurry has a Uk value of 1.35-1.60, and the de-aluminized residue is added in such an amount that the CaO thereof is 3-15% of the bauxite.

9. A method for preparing alumina from a medium/low grade bauxite according to any of claims 1-7, characterized in that: in step b), the Bayer red mud wet processing is conducted at 200-300'C for 5-120 minutes, wherein the recycling alkaline solution has a Nk of 30-260g/L and an A1 2 0 3 content of equal to or less than 20g/L, the lime is added in such an amount that the discharged red mud has a C/S ratio of 0.5-2.5, and the Bayer red mud is added in an amount of 10-350g per L of the recycling alkaline solution.

10. A method for preparing alumina from a medium/low grade bauxite according to claim 9, characterized in that: in step b), the recycling alkaline solution has a Nk of 90- HAgw\Interwoven\NRPo tbl\DCC\GW\5597482_ ldoc-4/10/2013 - 16 220g/L and an A1 2 0 3 content of equal to or less than 20g/L, the lime is added in such an amount that the discharged red mud has a C/S ratio of 1.0-2.1, and the Bayer red mud is added in an amount of 200-350g per L of the recycling alkaline solution.

11. A method for preparing alumina from a medium/low grade bauxite according to any of claims 1-7, characterized in that: before the recycling alkaline solution is fed into step b), caustic alkali is added into the recycling alkaline solution.

12. A method for preparing alumina from a medium/low grade bauxite according to any of claims 2-7, characterized in that: the de-aluminization is conducted at a temperature of 40-1 10 C for 10-180 minutes, wherein the lime is added in such an amount that a C/A ratio of 1.5-2.5 is obtained.

13. A method for preparing alumina from a medium/low grade bauxite according to any of claims 2-7, characterized in that: the de-aluminized slurry is separated by pressure filtration or sedimentation.

14. A method for preparing alumina from a medium/low grade bauxite according to any of claims 1-7, characterized in that: both step a) and step b) can be done by a Bushing Preheating-Retention Tank digestion technique, a Tube digestion technique, a Bushing preheating-autoclave digestion technique or a double stream digestion technique.

15. A method for preparing alumina from a medium/low grade bauxite according to claim 1, substantially as hereinbefore described.