Experimental Research On Preparation Of Polyaluminium Chloride From High-alumina Fly Ash

In this dissertation, a new and more effective technique was developed forpreparation of polyaluminium chloride(PAC)polymer from high-alumina fly ash atlower temperature than in other processes.The main components of high-alumina fly ash are SiO2 (48.70wB % ) andAl2O3(40.37wB%). Mullite (Al6Si2O13) crystal and aluminosilicate glass are themajor phases. The aluminium in high-alumina fly ash exists in the form ofaluminosilicate glass and mullite.The technique adopted here is to calcinate high-alumina fly ash and Na2CO3,changing the mullite and aluminosilicate glass into nepheline (NaAlSiO4). This is inturn easily dissolved in a dilute solution of HCl, precipitating out H2SiO3 and leavingAlCl3 solution. Thus it is possible easily to separate the main components Al2O3 andSiO2 in the fly ash materials.Experimental analysis of the thermal decomposition reaction of mulite andaluminosilicate glass phases shows that any superfluous Na2CO3 helps to reduce thereaction temperature. The surplus Na2CO3 will later react with nepheline to formalkaline nepheline [(Na2O)0.33NaAlSiO4].The optimal reation condition wasdetermined by the experiments as follows: the ratio of the high-alumina fly ash toNa2CO3 is 1.8:1 by mass,and that the optimal temperature is 850℃ for 90min.The dissolution reaction of nepheline in HCl solution is also thermodynamicallyanalyzed. At ambient temperature,the amount of HCl used in the reaction wasdetermined to be the only factor with a major effect on the reaction. 1.6L of HClsolution with a volume concentration of 80% is needed for 100g high-alumina flyash. Experiments also determined that under the above optimal conditions, up to97.63wB% of Al2O3 can be extracted from the high-alumina fly ash.The behavior of the Al3+ in the solution was studied, it was found that Al3+ can easilybe separated from H2SiO3 in a solution with pH less than 2.The filtered solution obtained from the nepheline and HCl solution reaction is useddirectly to preparate PAC of 0.2mol/L. The optimal experimental conditions forpreparing the PAC of 0.2mol/L were confirmed in laboratory studies. The PACproduct, with 89wB% of Al13 also contains a small amount of silica gel (SiO2 0.93 wB%) and ferric hydroxide (Fe2O30.28 wB%).Comparison shows that the PAC prepared using the technique described above ismuch better than the other 7 polymers popularly used at present to purify water, andeven better than the PAC (with the same cencontration) made from pure AlCl3solution. Thermodynamics shows that the silica gel and ferric hydroxide in the PACact to promote coagulantion, considerably enhancing its effection in treating water.