Study On The Process Of Synthesizing Ultra-fine Al(OH)₃ And Al₂O₃ From CFB Ash

The combustion technique of circulating fluidized bed (CFB) developed in the last 20 years is a new environmentally friendly technique. At present, China has become the largest CFB market in the world. Circulating fluidized bed technique can produce a large quantity of ash characterized with lower combustion temperature and higher activity compared to ordinary ash. However, few reports have been revealed about comprehensive utilization of CFB ash, especially concerning the research on advanced technique and highly profitable products, both at home, and abroad. Our country is a major country for the production and consumtion of alunmina. Bauxite mine is in short supply to satisfy production need, although China enjoyes the abundant reservation of this resource. The production of nano alumina in our country can not meet the need of domestic market.So,research on the preparation of ultra-fine or nano Al(OH)3 and Al₂O₃ is significant to strengthen this line of industry. Our work is focused on the process of synthesizing ultra-fine Al(OH)3 and Al₂O₃ from CFB ash with the purpose of realizing the maximum improvement of added value and resource recovery of CFB ash.According to the character of CFB ash, based on the process of extraction of Al, two systems, alkaline and acid systems, are developed to investigate a reasonable and optimal way of synthesizing ultra-fine Al(OH)3 and Al₂O₃ from CFB ash. The properties of the final products are investigated by means of BET,XRD,TG/DTA,TEM.The analysis results of CFB ash from Zhungeer power plant show that the content rate of Al₂O₃ is as high as 42%, the Al₂O₃ is active and in a noncrystal phase. The SEM graph illustrates that the shape of ash is irregular. The average particle size is 3.32um and D50=1.85um, tested by laser granularity apparatus.Two systems are developed. The alkaline system is acquired by the following steps: first , extract Al and Fe by getting rid of Si using HCl, and AlCl3 solution is achieved, and then transform the solution to raw Al₂O₃ through the process of concentration,crystallization and calcination. Second, purify raw Al₂O₃ using NaOH to remove Fe. The acid system is obtained by extracting and purifying AlCl3 solution (acquired by the same technique mentioned above) with TBP as extraction agent to eliminate impurities.Ultra-fine Al(OH)3 and Al₂O₃ are attained by carbonation technique in the alkaline system with sodium aluminate solution (SA). The effects of main factors, such as the velocities of CO2 and air, concentration of SA, reaction time, amount of PEG4000, on the specific surface area of the final product, are investigated in detail by performing single factor experiment. Three conclusions can be made: first, in the carbonation process, moderate velocities of CO2 and air neither too high nor too low are in favor of synthesizing fine products; second, when the concentration of SA is≥1.2mol/L, the reaction time will be extended and not proportionate to the concentration of SA, and the efficiency will be decreased and the equipment will be blocked. The last one is that, the final value of PH can change the crystal form of Al(OH)3, the higher the PH is, the higher the specific surface of area pseudo-boehmite will be. The results from orthogonal experiments suggest that the mixture of PEG200 and PEG 20 000 with the ritio of 1:1 leads to the best dispersive effect, which can be attributed to the chimeric adsorption effect. To abtain products with small particle size, the lowest temperature and the shortest cooling time should be choosen on the premise of proper crystalline form. The optimum conditions to prepare Al(OH)3 are listed as follow: velocities of CO2 and air should be 40 ml/min and 320ml/min, respectively; concentration of SA should be 0.9mol/L and PEG200 and PEG 20 000 mixed at the ritio of 1:1. The Al(OH)3 abtained is in the form of irregular flake, the size scale of which is less than 50 nm in all 3 single-dimension. Due to agglomeration during drying and calcining, the optimum conditions to prepareγ-Al₂O₃ is different from that to produce the Al(OH)3, the velocity of air should be 240ml/min; the concentration of SA should be 0.9mol/L and the calcining temperature should be 800℃and kept for 2h and then cool in the air. Fibrousγ-Al₂O₃ in size of 5nm×40nm and with big specific surface area of 205m2/g is prepared.Ultra-fine Al₂O₃ is synthesized by direct precipitation, in the acid system of purified AlCl3 solution, ammonia is used as precipitant. The influences of involved factors on particle size,such as concentration of ion in the liquid phase, amount of disperser, temperature, PH, ect are investigated by single factor experiments. XRD and IR are used to identify the product. SSA and TEM are used to characterize the product. The results indicate that the way of adding ammonia is the key factor. The change of PH value is inspected when the adding speed of ammonia remains constant, for PH can indicate the status of reaction. It's found that the curve of PH can be divided into three steps. According to the thermodynamics and dynamics, the three steps correspond to the 3 stages during the precipitation of Al(OH)3: pre-inducement and inducement , nucleation and growth, growth, respectively. During the second step, 2 waves can be seen on the curve, suggesting that adding ammonia at a constant rate can lead to second nucleation and growth. To obtain the excellent products, uniform and instantaneous nucleation is necessary and this requirement can be approached by controlling the way of adding ammonia. The major principle is to divide the ammonia adding rate into 2steps: slow-fast. The critical adding amount depends on the conditions of AlCl3 solution and concentration of ammonia, which can be verified by experiments. The optimal conditions are given as follow: concentrations of ammonia and AlCl3 should be 3.75mol/L and 2mol/L, respectively, temperature should be 30℃, the amount of 5% PEG4 000 solution should be 3ml and the final PH should be 5.The rate of ammonia adding fellows: slow-fast. When the volume of AlCl3 solution is 20ml, the reference critical amounts of ammonia is 13ml. The synthesized amorphism Al(OH)3 can transform to columnarγ-Al₂O₃ with a length diameter ratio of approximately 200nm/100nm and a large specific surface area of 201m2/g by sintering at 850℃for 2h.Comparing the two systems, we can conclude that, the alkaline system takes advantage over the acid one, through several aspects illustrated as follow: in the alkaline system, NaOH is used to remove Fe. The process is simpler and the reagents cost less, compared with the acid system in which TBP is used as extractant; during the process of synthesizing ultra-fine Al(OH)3, in the alkaline system, CO2 which is safe and does no harm to human, is used for carbonation, and the filtrate mainly contains NaCO3 solution which is easy for recycle, in contrast, in the acid system, ammonia is used as precipitator which causes unfavorable and irritative scent and the filtrate mailnly consists of NH4Cl solution, which is difficult to recycle. Therefore, alkaline system is a preferred one.The purpose is to explore an effective way of preparing ultra-fine Al (OH) 3 and Al₂O₃ from CFB ash, based on the process of extraction of Al and characters of CFB ash. The technology in alkaline system can synthesize high quality product and also goes with a simple, economic and efficiant process with low pollution. A system of Al (OH) 3 and Al₂O₃ products can be developed to satisfy different needs by controlling the preparing conditions. It's significant to realize the maximum profits of resource recovery of CFB ash and the improvement of the combustion technique of CFB.