论文部分内容阅读
Modern aluminum production involves two independent industrial processes for the transition from the naturally occurring aluminum oxide ores to the extracted metal.
The smelting of pure aluminum requires high purity aluminum oxide, carbon and electrical power.The overall process in a typical smelter utilizing prebaked anodes in modern plants the incoming alternating current is transformed directly to DC at high voltages, (e.g. 440 or 880 V) and is fed to a line of cells connected in series. In this way the operation is essentially at constant current,although the individual voltage of each cell can be varied. The recent improvements in solid-state rectification systems have led to quite a dramatic change in this section of aluminum smelters. Not only has it reduced the required dimensions of the rectifying sites, but it has also increased the efficiencies of rectification and the relative costs.
Aluminum is now the largest non-ferrous metal and the production capacity to meet future requirements continues to expand. It is the third most abundant element in the earths crust being behind oxygen and silicon, and occurring to the extend of 8 wt％. Due to high reactivity,aluminum is rarely found in nature as element, but always in its oxidized form - most commonly in the form of aluminates and silicates. Within these compounds, aluminum occurs as the free oxide ( Al2O3 ), combined with water, or with some other compounds.
Historically, the first mention of metallic aluminum was in collective works of the First Century Roman author Gaius Plinius the elder. In this famous encyclopedia “Historia naturalis” which was a 37 volume collection of all fields of knowledge at that time.
Technical production of aluminum did not occur until the developments by St. Claire-Deville, a French schoolteacher, in 1854. He produced aluminum from melted sodium tetrachloroaluminate,NaAlCl4, by thermal reduction methods. The processes were still very expensive, and Deville received direct government subsidy for light metal production from the French Emperor Napoleon.
Cheaper commercial production of metallic aluminum started in 1889 with the electrolysis of a solution of aluminum oxide dissolved in molten Cryolite at about 975 ℃ - this being the birth of the present process. It is well known that aluminum is quite insoluble in most solvents. However,sodium hexafluoroaluminate (Na3AlF6), which occurred naturally in Greenland for electrolytic processes in that:
It is a relatively good solvent for alumina.
It has a higher decomposition voltage than that of alumina.
It has good electrical conductivity in the molten state.
It has a sufficiently low melting temperature.
It does not react with aluminum and carbon to any appreciable extend.
It forms quite a fluid melt.
It density is lower than that of aluminum when both are liquid.
It has a reactively low vapor pressure.
With respect to 30 years experience, Iran has decided to develop aluminum smelters in Iran, and using the modern technology. In this regard the general plan for aluminum industry including smelter, anode plant, alumina and petroleum coke was considered by Iranian Ministry of Industries and Mines. Based on this general plan, Iranian Aluminum Companies started to negotiate with Chinese and European companies in order to get advanced technologies for above-mentioned projects. Some of these projects are contracted now with Chinese Companies and some are under negotiation.
Saving the energy is one of the most important factors in aluminum production, because 30 ％ of production costs is electrical energy, so most of the smelters in the world decided to decrease the energy consumption and improve the current efficiency.
Based on the theoretical and experimental factors, we are able to produce primary aluminium with suitable bath composition, decreasing bath temperature and modern electrolytic pots and finally according to the instruction of supervisor professor Qiu Zhuxian of the Northeastern University in China, improve the current efficiency. In Iranian smelter, 10 pots were selected and 5 ％ magnesium fluoride added to the bath, in order to decrease the bath temperature. Also we covered these pots by special aluminium covers, in order to decrease heat losses of the pots.Within 6 months, we could reach to a good result. The result was improving 2 ％ of current efficiency, which causes lower consumption of electricity .In our smelter, the functions of the pots are controlled by computerized system and we can send the data of the pots to the system and all the functions can be analyzed automatically. During this period we met no special troubles.