AU2007356401A1

AU2007356401A1 - A method for producing alumina with middle and low grade bauxite

Info

Publication number: AU2007356401A1
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: 2009-01-15
Anticipated expiration: 2027-12-21
Also published as: AU2007356401B2; RU2010104624A; CN100532262C; WO2009006772A1; CN101113001A; RU2478574C2

Description

rru / i oUU'or 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 5 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 10 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 15 and a lower energy consumption, but suffer from a low recovery of alumina (only about 70%) and a poor ulitization 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 20 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.I, see Technology for Preparing Alumina, edited by Yang Chongyu, Metallurgical Industry 25 Publisher, Beijing, 1982, p307. 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 30 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 a high alkali concentration of 400-500g/L calculated as Na 2 0, as is difficult to realize in industrial scale; 3) the process flow is complex. Thus, this Bayer hydration process is difficult to put into 35 practice in industrial scale. In order to overcome these shortcomings mentioned above, this invention provides a totally wet method for preparing alumina by processing a low grade bauxite. This method has advantages in that the operation temperature is lower, the alkali concentration is lower and the resulting residue is less. Thus, this method can 40 decrease energy consumption significantly and be easily conducted in industrial scale. Summary of the Invention The objective of this invention is 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 5 is easy to conduct comprehensive utilization, so as to overcome the shortcomings of the prior art. The inventive method is realized by the following technical solutions. In one embodiment according to this invention, provided is a method for preparing alumina from a medium/low grade bauxite, wherein the bauxite is firstly 10 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. According to one aspect of this invention, provided 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 15 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 20 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. 25 The inventive method has a suitable process flow and is easy to operate. For any currently available series-parallel alumina preparing plant, the incorporation of only one set of wet digestion system will 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. 30 The inventive 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 is improved and the discharged red mud is reduced. The inventive method is characterized in that the discharged red mud has a lower amount of Na 2 0 and thus facilitates the comprehensive use of red mud. 35 The inventive method is suitable for use in a plant which prepares alumina from a medium/low grade bauxite. The inventive method has positive effects on improving the utilization of bauxite resource and reducing the energy consumption and alkali consumption in alumina preparation, and thus has a good perspective. 40 Brief Description of the Drawings Fig.I is the flow chart of the Bayer hydration process of the prior art; Fig. 2 is the flow chart of a totally wet method according to this invention; and Fig.3 is the flow chart of another totally wet method according to this invention. 2 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 5 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, "ak" refers to the molar ratio of sodium hydroxide and alumina in a solution. 10 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 15 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 aspect of this invention, a medium/low grade bauxite, a recycling mother liquor and a de-aluminized residue, as additive for digestion, are 20 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 Ck value of 1.35-1.60, and the de-aluminized 25 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. Accroding to another aspect of this invention, the Bayer digestion can be realized by a Bushing Preheating-Retention Tank digestion technique, a Tube digestion 30 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 aspect of this invention, the Bayer digested slurry is futher 35 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 40 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 5 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 3 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 from the de-aluminization is separated by means such as pressue 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 inventive embodiments, the Bayer red mud wet processing is 5 carried out at a low temperature and a low alkaline value, as makes 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 will eliminate the need for a sintering system having a high energy consumption and a complex process flow. Thus, the energy consumption needed to 10 prepare alumina can be significantly reduced. According to the inventive embodiments, 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 15 wet processing. The resultant de-aluminized residue can serve as additive for Bayer digestion. According to the inventive embodiments, 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 is improved and the discharged red mud is reduced, as 20 facilitates the comprehensive use of red mud. The inventive method is suitable to prepare alumina from a medium/low grade bauxite. The inventive method has positive effects on improving the utilization of bauxite resources and reducing the energy consumption and alkali consumption in alumina preparation, and thus has a good perspective. 25 Next, this invention will be further illustrated by the following specific examples. The skilled in the art shall recognize that this invention is not limit to these examples. Examples Example 1 30 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 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 35 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, whererin a recycling alkaline solution with a concentration of 130g/L 40 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 1.3. A portion of the digested liquor obtained from the Bayer red mud wet processing 5 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 /

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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 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, whererin 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 5 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 10 digestion is conducted at 270'C for 60minutes. 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 2600C for 90 minutes, whererin the recycling alkaline solution used has a concentration of 130g/L and lime is added in a total amount such that the CaO / Si0 2 ratio is 0.9. The digested red mud has an A/S ratio of 0.60 and a 15 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. 20 13

Claims

1. A method for preparing alumina from a medium/low grade bauxite, characterized in that it includes the following steps: a) Bayer digesting the bauxite; and b) wet processing a Bayer red mud obtained from step a).

2. The method for preparing alumina from a medium/low grade bauxite according to claim 1, characterized in that: at least a portion of a red mud digested liquor obtained from step b) is subjected to de-aluminization to give a de-aluminized residue and a recycling alkaline solution, whererin the de-aluminized residue is used as at least part of a digestion additive in step a), and the recycling alkaline solution is fed into step b) to process the next batch of Bayer red mud.

3. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: the de-aluminization is done in a pressure-filtration manner.

4. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: 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.

5. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: the recycling alkaline solution has a Nk of 90-220g/L and an A1 2 0 3 content of equal to or less than 20g/L.

6. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: the recycling alkaline solution has a Nk of 100-190g/L and an A1 2 0 3 content of equal to or less than 20g/L.

7. The method for preparing alumina from a medium/low grade bauxite according to claim 4, characterized in that: a caustic alkali is added into the recycling alkaline solution before the recyling alkaline solution is fed into step b).

8. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: step a) is conducted at a temperature of from 240 to 300 'C with the addtion of a recyling mother liquor having a Nk of 180-280g/L.

9. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: step b) is conducted under conditions of a temperature of 200-300*C, an alkali concentration of 30-260g/L and an A1 2 0 3 content of equal to or less than 20g/L.

10. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: step b) is conducted under conditions of a temperature of 200-300'C, an alkali concentration of 90-220g/L and an A1 2 0 3 content of equal to or less than 20g/L.

11. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: step b) is conducted under conditions of a temperature of 200-300*C, an alkali concentration of 100-190g/L and an A1 2 0 3 content of equal to or less than 20g/L.

12. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: step b) is conducted under conditions of a temperature of 200-280*C, an alkali concentration of 30-260g/L and an A1 2 0 3 content of equal to or less than 20g/L.

13. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: step b) is conducted under conditions of a temperature of 200-280'C, an alkali concentration of 90-220g/L and an A1 2 0 3 content of equal to or less than 20g/L.

14. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: step b) is conducted under conditions of a temperature of 200-280*C, an alkali concentration of 100-190g/L and an A1 2 0 3 content of equal to or less than 20g/L.

15. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: step b) is conducted under conditions of a temperature of 240-270'C, an alkali concentration of 30-260g/L and an A1 2 0 3 content of equal to or less than 20g/L.

16. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: step b) is conducted under conditions of a temperature of 240-270*C, an alkali concentration of 90-220g/L and an A1 2 0 3 content of equal to or less than 20g/L.

17. The method for preparing alumina from a medium/low grade bauxite according to 2 claim 2, characterized in that: step b) is conducted under conditions of a temperature of 240-270*C, an alkali concentration of 100-190g/L and an A1 2 0 3 content of equal to or less than 20g/L.

18. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: step b) is conducted with the addtion of lime.

19. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: the de-aluminization is conducted with the addtion of lime in such an amount that the C/S ratio is 0.5-2.5.

20. The method for preparing alumina from a medium/low grade bauxite according to claim 2, characterized in that: the de-aluminization is conducted with the addtion of lime in such an amount that the C/S ratio is 1.0-2. 1.

21. The method for preparing alumina from a medium/low grade bauxite according to any of claims 1-20, characterized in that: a Bayer red mud from step a) is back washed with a discharged red mud wash liquor from step b) and then fed to step b).

22. The method for preparing alumina from a medium/low grade bauxite according to claim 21, characterized in that: the wash liquor obtained from the back washing is de-aluminized and then fed into the recycling alkaline solution.

23. The method for preparing alumina from a medium/low grade bauxite according to any of claims 1-20, characterized in that: a Bayer red mud from step a) is back washed with a portion of a red mud digested liquor from step b) and then fed to step b).

24. The method for preparing alumina from a medium/low grade bauxite according to claim 23, characterized in that: a portion of the wash liquor obtained from the back washing is de-aluminized and then fed into the recycling alkaline solution, and the other portion of the wash liquor obtained from the back washing is fed into the recycling mother liquor.

25. The method for preparing alumina from a medium/low grade bauxite according to any of claims 1-20, characterized in that: both step a) and step b) can be realized by a Bushing Preheating-Retention Tank digestion technique, a Tube digestion technique, a Bushing preheating-autoclave digestion technique or a double stream digestion technique.

26. The method for preparing alumina from a medium/low grade bauxite according to claim 21, characterized in that: both step a) and step b) can be realized by a Bushing Preheating-Retention Tank digestion technique, a Tube digestion technique, a Bushing 3 preheating-autoclave digestion technique or a double stream digestion technique. 4