| Coal ash is an industrial waste residue from power plant, and power industries' development in China have directly led to the progressive increase in its discharge. The waste residue not only has a large discharge, but also is harm to environment. The comprehensive utilization of fly ash has been receiving increasing attention around the world. At present, the fly ash has been widely used in building materials, construction engineering, road building, agriculture and other fields, but the recovery of a useful element, is still at a research stage. Gallium is the indispensable materials in high technology fields, and the demand for it has increased in recent years. In the fly ash, the content of gallium is 12～230μg·g-1,so it has great value of extraction. However, there have not been suitable recycling methods yet. Recoveries of gallium aqueous solutions can be mainly achieved by solvent extraction which is frequently limited to industrial application and is hard to recycling gallium from fly ash because of several economical technical and environmental constraints, including low extraction efficiency, high consumption of extraction, complicated extraction process and so on. Thus, new separation technologies which can separate and recover gallium from different primary and secondary sources by high-effective, low-cost and environmental friendly way are of great importance. In addition, the Al2O3 content of fly ash is more than 30% in common, so it can be taken as the alumina-making raw material, which is a better method to reuse the waste residue high effectively, reduce utilizing bauxite and relax the Al2O3 requirement inland. In the thesis, the analysis, extraction and separation methods of gallium and aluminum in fly ash were studied.In this study, gallium may exist as stable anion CaCl4- based on the hydrochloric acid, under the same condition-NH-within the polyurethane foam can add the proton to form-NH2+-, so ion exchanges and association reactions can occur between associating. Therefore polyurethane foam plastic is be taken as the absorbent, and the adsorption and separation methods of gallium in fly ash have been carried on in the experiment. In this study, rhodamine B solvent extraction photometry has been established firstly to determine the gallium quantitatively. Through the single factor experiments and orthogonal tests on effects of various factors of the pretreatment process of fly ash and the adsorption and desorption processes of polyurethane foam, finally optimal process parameters of the extraction process of the gallium in coal ash have been obtained, that is:under the conditions that the mass ratio of the fly ash,anhydrous Na2CO3 and Na2O2 is 5:5:2 and the temperature is 700℃, ash samples have been kept for 10min in the way of alkali fusion; then ash samples have to go through the process of acid leaching by the concentration of 6mol·L-1 hydrochloric acid; whereafter ash samples have to be adsorbed at room temperature by using quantitative amount of foam and oscillating 2 hours under the concentration of 6mol·L-1 hydrochloric acid medium, and the adsorption rate is up to over 98%; finally, under the concentration of 0.5mol·L-1 NH4Cl solution and the room temperature, tha gallium in ash samples can be basicly eluted after oscillation for 45 min. And the interfering ions have little effect in this process. After two times of foam absorption process, the gallium in fly ash can be completely adsorbed, and after each absorption process it can be eluted completely at least by three times of desorption process. Besides foam can be used repeatedly after washed. The experiments have finally got a new significant method in the recovery of gallium in fly ash.Acid leaching process of extracting alumina from fly ash has been proposed on the basis of analysis and discussion of existing extraction methods, and fluoride substituent-EDTA volumetric method has been founded to determine the alumina quantitatively above all. Through the single factor experiments and orthogonal tests on effects of various factors of the pretreatment process of fly ash with the addition of NaF and acid leaching process, finally optimal process parameters of the extraction process of the alumina in coal ash have been obtained, that is:under the conditions that the mass ratio of the fly ash and NaF is 10:2 and the temperature is 800℃, ash samples have been kept for 1h in the way of roast; then at a temperature of 100℃, the concentration of 6mol·L-1 hydrochloric acid and the ratio between liquid and solid that is 4:1, the acid leaching of alumina has been stirring for 1h. Under these conditions, the lixiviating rate of alumina is more than 65%. Significant and practical extraction process of alumina from coal ash by the acid leaching process has been gained in this experiment.The reaction mechanism of adsorption by polyurethane foam in the acidic conditions has been discussed preliminarily. The results show that the reaction follows the first reaction kinetics and can be described by the Langmuir-Hinshelwood model. The average activation energy of adsorption process is 54.78 kJ·mol-1.In the end, the acid leaching process of extracting both alumina and gallium from fly ash is proposed, and the selection of equipmen in this process and economic and other related aspects are discussed. Finally the practical production feasibility of this process has been proved. Therefore, the process has good prospects for industrial applications and will bring a very high social and economic benefits. It also proposed new ways of development and other directions in comprehensive utilization of coal ash. |
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