Title of Invention

A PROCESS FOR REMOVING CARBONATES IN THE PRODUCTION OF ALUMINA

Abstract

The present invention relates to a process for removing carbonates in the production of alumina, characterized in that 0.3 to 1.0 part by weight of carbonate seed crystals is added into a forced circulation evaporator or a flash tank so that the majority of the salt precipitated out by crystallization is anhydrous sodium carbonate and that a caustic alkali liquor with a concentration of 450-650g/l is added during the forced circulation or flash vaporization so that the caustic alkali (Na2Ok) concentration of the desalting solution reaches about 320 g/l and the ratio of the carbon alkali (Na2Oc) to the toal alkali (Na2OT) of the desalted solution is decreased to about 6%.

Full Text

A Process for Removing Carbonates in the Production of Alumina Technical field The present invention pertains to the technical field of salt removal in the production of alumina, The invention relates to a process for removing salts in the production of alumina according to the Bayer process. In particular, the invention relates to a process for removing carbonates in the production of alumina according to the Bayer process from bauxites, in particular from diasporite or boehmite. State of the art In the production process of alumina according to the Bayer process from bauxites, especially diasporite or boehmite, carbonates accumulate continuously in the cyclic solution of sodium aluminate, decreasing the circulation efficiency and the productivity of the production of alumina. When the accumulation reaches a certain extent, carbonates may precipitate out on the heat exchange surface of the apparatus such as dissolves, refined-liquor heat exchanger, evaporator and the like, causing the efficiency of the heat exchange to decrease (or the energy consumption to increase). A more grievous problem is that carbonates precipitate in conduits, valves and the like and clog the same, forcing the production process to be broken down. Therefore, there is a need to develop more effective processes for salt evacuation. There are five kinds of the existent techniques for removing carbonates in the art, i.e., aging desalination, side-stream causticization, causticization with barium salts, desalination by flash evaporation and desalination by forced circulation, 1 (1) Aging desalination In the production of alumina according to the Bayer process, the equilibrium concentration (the ratio of the carbon alkali (carbonated Na2O Na2OC) to the total alkali (total Na2O Na2OT)) of carbonates in the cyclic liquor (sodium aluminate solution) decreases as the concentration of the caustic alkali (caustic Na2O, Na2Ok) in the liquor increases and the temperature of the liquor decreases. In this process, after the decomposition mother liquor having a caustic alkali concentration of 14O~18O g/l is concentrated by evaporation, the resultant solution has a temperature of about 140˚C and a caustic alkali concentration of 230~280 g/l. Then, the solution is subjected to a flash evaporation by self-evaporating and a settlement for about 2 hours in a salt-settling tank to increase the caustic alkali (Na2OK) concentration to 240~-290 g/l and decrease the temperature to 8O~90˚C, thereby the aging desalination is carried out. After the underflow obtained from the salt-settling is then separated by filtering, the overflow from the salt-settling and the filtrate are delivered to the compounding system in the alumina production line, and the filter cake after being regenerated by causticization is circulated to the alumina production line. This process is poor in the desalting effect, and about 70% of the precipitated salts are fine particles of aqueous sodium carbonate (Na2CO3.nH2O) which has poor performance in settling and filtering. Therefore the process is no longer suitable in the practice. (2) Side-stream causticization The secondary washing liquor (A12O3 = 20~40 g/l, Na2Oc = 3 ~5 g/l) from the red mud settlement and the lime slurry (about 200 g/l of CaO) are introduced simultaneously into a side-stream causticization tank for causticization. Na2CO3 + Ca(OH)2 - 2 NaOH + CaCO31 The products of the causticization reaction are NaOH and solid CaCO3. The causticized slurry is separated by settling. The resultant underflow is introduced into a secondary washing tank for the red mud settlement, while the resultant overflow is introduced into a first washing tank for the red rnud settlement This process has disadvantages to that: (I) It requires a separate side-stream causticization procedure for desalination, which is relatively complicated and expensive in the construction investment and operation cost; The concentration of the secondary washing liquor is relatively low (3~5 g/1 of Na2OC)) so that the desalting effect is poor; and (3) The addition of lime into the secondary washing liquor causes formation of solid tricalcium aluminate (3CaO • A12O3) from alumina in the washing liquor and thereby results in losing the useful component - alumina. Due to aforementioned reasons, utilization of this process should not be promoted. (3) Causticization with barium salts A barium salt (BaO or Ba(OH)2) is introduced to the decomposition mother liquor or circulation mother liquor so that the causticization reaction of the carbon alkali in the solution with the barium salt occurs, Na2CO3 + Ba(OH)2 → 2 NaOH + BaCO3 ↓ The reaction products are caustic alkali (NaOH) and solid barium carbonate (BaCO3) which are then separated by settling and filtering. The separated caustic alkali liquor is fed into the alumina production line and reused, The solid barium salt is regenerated by calcining so that the barium salt (BaO) is recovered and reused. This process suffers not only from the high price of the barium salt, but also from the environmental pollution caused by the calcination and recovery procedures of the barium salt. It is been out of use after industrial trials for many years abroad. (4) Desalination by flash evaporation After the decomposition mother liquor having a caustic alkali concentration of 140-180 g/1 is concentrated by evaporating, it is introduced into a flash tank for further to a concentration to 240~90 g/l with a flash cooling concentration process. Desalination is effected thereby. This process has the disadvantages in that: (1) The solution after the flash evaporation has a caustic alkali (Na2Ok) concentration of only 240~290 g/1 and a ratio of the carbon alkali to the total alkali in the solution of 9.0~11.0%, which is below the optimal desalting concentration (Na2Ok = 320 g/1) with the ratio of the carbon alkali to the total alkali in the solution being about 6%. That is to say, the desalting effect is poor; (2) About 60% of the precipitated crystalline salts are fine particles of aqueous sodium carbonate (Na2CO3 . n H2O) having poor performance in salt settling and filtering, and about 40% of the crystalline salts are circulated directly back to the production line during the separation procedure; and (3) The moisture level of the filter cake from the salt filtering is about 45%. (5) Desalination by forced circulation After the decomposition mother liquor having a caustic alkali concentration of 140 ~ 180 g/1 is concentrated by evaporating and flash evaporating, the caustic alkali concentration reaches 240~290 g/1 which is below the desired desalting concentration (Na2OK = 320 g/1). In this process, a portion of the solution is split out from the flash tank and introduced into a forced circulation evaporator for ultra-concentration, so that the caustic alkali concentration of the solution reaches about 320 g/l and the ratio of the carbon alkali to the total alkali reaches about 6%. Though this process has a good precipitation effect for the crystalline salts and overcomes the major disadvantages of the four desalting processes mentioned above, it still has the disadvantages in that about 40% of the precipitated salts are fine particles of aqueous sodium carbonate, about 25% of the crystalline salts are circulated directly back to the production line during the settling and filtering, and the salt filter cake also has a liquor content of about 40%. The essential contents disclosed in the prior art (for example CN 03802511.6 and etc.) lie in that condensation water is introduced into the feedstock inlet (outlet) pipe of the flash tank of the high pressure solubilizing system to increase the solubility of carbonates in the sodium aluminate solution and prevent carbonates from precipitating during the flash concentration, and thus prevent the conduits and valves from scaling and clogging during the flash evaporation. The essential features of the present invention lie in that a high concentration of caustic alkali liquor (Na2OK)= 450~650 g/1) is introduced into the forced circulation evaporator or flash tank of the vaporization system so that the caustic alkali (Na2OK)) concentration of the desalting solution reaches about 320 g/1 and the equilibrium concentration of the carbon alkali (a weight ratio of the carbon alkali Na2OC: to the total alkali Na2OT) of the desalting solution reaches about 6%. Thereby carbonates can be precipitated out from the solution and the object of removing carbonates from the production line can be achieved. In the process according to the present invention, the carbon alkali that is not reacted in the solubilizing process is evacuated. Thereby, the solubilizing effect is improved and the productivity and the circulation efficiency are increased. Moreover, it effectively prevents carbonates from further accumulating in the production line and prevent carbonates from precipitating on the heating surface of heat-exchange equipment used for the procedures such as the refined liquor heat exchange and evaporation, thereby the heat exchange efficiency is increased. On the contrary, the disclosed patents prevent carbonates from precipitating by adding condensation water rather than removing the salts from the production line. This brings about negative effects as compared with the present invention. Moreover, the solution is further diluted by the condensation water is added, causing the decomposition productivity and the circulation efficiency to further decrease and the quantity of the evaporating water to increase (which means that the energy consumption and the production cost are increased). Furthermore, addition of dilution water is not able to prevent carbonates from accumulating further in the production line; namely the problem on removal of carbonates is not basically solved. When the carbon alkali is accumulated to a given extent, carbonates may precipitate out during solubilization and flash evaporation, or more condensation water needs to be added. Content of the invention The present invention aims at overcoming the deficiencies of the five existent desalting techniques in the alumina production process and proposes that carbonate (Na2CO3)) seed crystals and a high concentration of caustic alkali liquor are introduced during the desalination by forced circulation and flash evaporation to achieve the following objects; (1) The desalting effect is improved by decreasing the ratio of the carbon alkali to the total alkali in the desalting solution to about 6%; (2) About 60% of the salts are precipitated out as coarser particles of anhydrous sodium carbonate (Na2CO3) so that the settling and filtering performance of the salts are improved; and (3) The moisture level of the salt filter cake is decreased (to about 35%). The process of the present invention is achieved by adding solid carbonate seed crystals (Na2CO3) and about 450 ~ 650 g/1 of caustic alkali liquor into the existent forced circulation evaporator or flash tank. The amount of the seed crystals added varies with the carbonate content in the desalting solution and the impurity content of the solution, and the seed crystal coefficient (i.e., the ratio of the added carbonate seed crystals to the carbonates in the solution) is typically 0.3~1.0. This is the major measure to ensure that the salts precipitate out by crystallization as anhydrous sodium carbonate. The amount of the high concentration of caustic alkali liquor added varies with the caustic alkali concentration of the solution in the forced circulation evaporator or flash tank. It is added in such an amount that the concentration of the caustic alkali (Na2OK) of the desalting solution reaches the optimal concentration (Na2Ok = 320 g/1) for desalting. Embodiment of the two measures mentioned above can decrease the ratio of the carbon alkali (Na2Oc) to the total alkali (Na2OT) in the desalting solution to about 6%, and enable about 60% of the salts to precipitate out by crystallization as coarser particles of anhydrous sodium carbonate which has good settling and filtering performance and only about 5% thereof are circulated directly back to the production line during the separation procedure by settling and filtering. The liquor content of the salt filter cake is about 35%. In comparison with the desalting techniques in the art, the process according to the present invention has the advantages in that; (1) the desalting effect is good as the ratio of the carbon alkali (Na2OC) to the total alkali in the desalting solution is decreased to about 6%; (2) about 60% of the salts precipitate out by crystallization as anhydrous sodium carbonate which has good settling and filtering performance and only about 5% thereof are circulated directly back to the production line during the separation procedure by settling and filtering, and the liquor content of the salt filter cake is about 35% or less; (3) the circulation efficiency for the alumina production is increased; and (4) the precipitation of carbonates during the concentrating procedures such as high pressure solubilization and evaporation is avoided, which is advantageous to a continuous and stable operation of the alumina production line. Brief description of the drawings Figure 1 is a schematic diagram showing the desalting process with a flash tank according to the present invention, and Figure 2 is a schematic diagram showing the desalting process by forced circulation according to the present invention, wherein: 1: flash tank; 2: salt-settling tank; 3; tank for the high concentration of caustic alkali liquor; 4; pump for the salt seed crystals; 5: pump for the high concentration of caustic alkali liquor; 6; crystallizing compartment; 7: heating chamber; 8: salt slurry pump; 9: pump for the salt settling underflow; 10: salt filter. Specific embodiment modes A pump (4) for the salt seed crystals is additionally disposed at one side of the existent salt-settling tank (2) in the production line, and a tank (3) for the high concentration of caustic alkali liquor and a pump (5) for the high concentration of caustic alkali liquor are additionally disposed close to one side of the desalting system. The seed crystals and the high concentration of caustic alkali liquor are introduced into the flash tank (1), or introduced into the crystallizing compartment (6) or the heating chamber (7) of the forced circulation evaporator, by using the aforementioned two pumps via the conduits so that the caustic alkali concentration of the desalting solution reaches 320 g/1 and about 60% of the salts precipitated out by crystallization are the coarser particles of anhydrous sodium carbonate which has good settling and filtering performance. The salt slurry is pumped via the conduits to the existent salt settling tank (2) in the production line by using the existent salt slurry pump (8) in the production line to subject to settling and separating. The separated underflow is pumped via the conduits to salt filter (10) by using the existent pump (9) for the salt settling underflow in the production line to subject to separation by filtering. The salt settling overflow with a ratio of the carbon alkali to the total alkali of about 6% and the filtrate from the salt filtration are pumped to the alumina production line for compounding. The filter cake from the salt filtration is regenerated by causticizing and the resultant solution is pumped to the mother liquor evaporating tank or the compounding tank for the circulating mother liquor in the alumina production line. When the caustic alkali concentration reaches 320 g/1 (which is difficult to be achieved by flash evaporation) after the desalting solution is ultra-concentrated by the forced circulation, it is only necessary to add seed crystals and the addition of the high concentration of caustic alkali liquor can be dispensed with. Example 1 After the decomposition mother liquor is concentrated by evaporating, the solution introduced into the forced circulation evaporator or flash tank has a caustic alkali (Na2OK) concentration of 220 g/1, a total alkali concentration of 250 g/1 and a ratio of the carbon alkali (Na2OC) to the total alkali (Na2OT) of 12%. After further concentration by forced circulation or flash evaporation, the solution has a caustic alkali concentration of 250 g/1, a total alkali concentration of 279.3 g/1, and a ratio of the carbon alkali (Na2OC) to the total alkali (Na2OT) of 10,6%. During the concentration, 8.0 g/1 of carbonate crystals is precipitated out, and about 60% of the crystalline salts are aqueous sodium carbonate (Na2COC • nH2O). During the separation by settling and filtering, about 40% of the crystalline salts are circulated directly back to the production line, so that the salt (Na2CO3) was practically removed by only about 4.8 g/1. According to the present invention, about 15.4 g of the carbonate (Na2CO3) seed crystals (the seed crystal coefficient of 0.3) and about 0,263 1 of a caustic alkali liquor having a concentration of 550 g/1 are added to a liter of the desalting solution, so that the caustic alkali concentration of the desalting solution is controlled to be 320 g/1, the total alkali concentration is 340.4 g/1 and the ratio of the carbon alkali (Na^) to the total alkali in the solution is about 6%, About 12.1 g of the salt (Na2CO3) is precipitated out by crystallization. About 60% of the crystalline salt is anhydrous sodium carbonate (Na2CO3) and about 5% thereof is circulated directly back to the production line during settling and filtering. Therefor, the practical salt removal is about 11.5 g/1 and liquor content of the salt filter cake is about 35%. Example 2 After the decomposition mother liquor is concentrated by evaporation, the solution introduced into the forced circulation evaporator or flash tank has a caustic alkali (Na2OT)concentration of 230 g/1, a total alkali concentration of 258.4 g/l and a ratio of the carbon alkali (Na2OC) to the total alkali (Na^) of 11%, After further concentration by forced circulation or flash evaporation, the solution has a caustic alkali concentration of 260 g/1, a total alkali concentration of 288,9 g/1, and a ratio of the carbon alkali (Na2O3) to the total alkali (Na2OT) of about 10%. During the concentration, 5.6 g/1 of carbonate crystals is precipitated out, and about 60% of the crystalline salts are fine particles of aqueous sodium carbonate. During the settling and filtering, about 40% of the crystalline salts are circulated directly back to the production line, so that the direct salt removal is about 3.4 g/1 and the liquor content of the salt filter cake is about 45%. According to requirements of the present invention, about 14.5 g of the carbonate seed crystals (the seed crystal coefficient of 0.3) and about 0.241 of a caustic alkali liquor having a concentration of 550 g/1 are added to a liter of the desalting solution, so that the caustic alkali concentration of the desalting solution reaches 320 g/1, the total alkali concentration is controlled at 340,4 g/1 and the ratio of the carbon alkali to the total alkali in the solution is about 6%. 9.9 g/l of the salt is precipitated during the procedure and about 5% thereof is circulated directly back to the production line during settling and filtering. Therefore, the practical salt removal is about 9.4 g/1 and liquor content of the salt filter cake is about 35%. Example 3 After the decomposition mother liquor is concentrated, the caustic alkali (Na2OK) concentration is 245 g/1, the total alkali concentration is 272.2 g/1, and the ratio of the carbon alkali (Na2OC) to the total alkali is 10%. After said solution is further concentrated in the forced circulation evaporator or the flash tank, the caustic alkali concentration reaches 280 g/1, the total alkali concentration reaches 307.7 g/l and the ratio of the carbon alkali (Na2OC) to the total alkali (Na2OT) is 9.0%. During the procedure, 5.78 g/1 of carbonate is precipitated out by crystallization, and about 60% thereof is aqueous sodium carbonate (Na2CO3. n H2O). During the settling and filtering, about 40% of the precipitated salt are circulated directly back to the production line, so that the practical salt removal is about 3.5 g/I and the liquor content of the salt filter cake is about 45%. According to the present invention, about 46.5 g of the carbonate (Na2CO3) seed crystals (the seed crystal coefficient of 1.0) and 0,15 1 of a caustic alkali liquor having a concentration of 550 g/1 are added to a liter of the desalting solution, so that the caustic alkali concentration of the desalting solution reaches 320 g/1, the total alkali concentration is 340.4 g/1 and the ratio of the carbon alkali to the total alkali in the solution is about 6%. 11.0 g/1 of carbonate (Na2CO3) is precipitated out during the procedure and about 5% thereof is circulated directly back to the production line during settling and filtering. Therefore, the practical salt removal is about 10.5 g/I and liquor content of the salt filter cake is about 35%. Example 4 After the decomposition mother liquor is concentrated by evaporation, the caustic alkali concentration (Na2Ofc) is 250g/l, the total alkali concentration (Na2OK) is 277.8g/l, and the ratio of the carbon alkali (Na2OT) to the total alkali is 10%. After further concentration by the forced circulation and evaporation, the solution has a caustic alkali concentration of 320 g/1 which is difficult to be achieved by flash evaporation) and a total alkali concentration of 340,4 g/1. 22.4 g/1 of carbonate (Na2CO3) is precipitated out in the procedure, and the ratio of the carbon alkali to the total alkali in the solution is decreased to 6%. Although the amount of the precipitated salt has met the requirement for desalting in this case, about 50% of the precipitated salt crystals are still aqueous sodium carbonate ((Na2CO3) • nH2O). During the settling and filtering, about 25% of the salt are circulated directly back to the production line. Therefore, the practical salt removal is about 16,8 g/l, and the liquor content of the salt filter cake is about 40%, Using the inventive desalting process, it is not necessary to add caustic alkali liquor to the desalting solution, while addition of about 46.5 g carbonate (Na2CO3) seed crystals (the seed crystal coefficient of 1,0) to a liter of the desalting solution is required. The salt precipitated out during the procedure is 22.4 g/1, and the ratio of the carbon alkali to the total alkali in the solution is about 6%. About 50% of the precipitated salt is anhydrous sodium carbonate. Only about 5% of the salt is circulated directly back to the production line during the separation by settling and filtering, so that the practical salt (Na2CO3) removal is 21.3 g/1, and the liquor content of the salt filter cake is decreased to about 35%.

Documents:

876-CHE-2006 CORRESPONDENCE OTHERS.pdf

876-CHE-2006 CORRESPONDENCE PO.pdf

876-che-2006-abstract.pdf

876-che-2006-claims.pdf

876-che-2006-complete descriptino.pdf

876-che-2006-corresspandance others.pdf

876-che-2006-drawing.pdf

876-che-2006-form-1.pdf

876-che-2006-form-3.pdf

876-che-2006-form-5.pdf

876-che-2006-priority document.pdf