单项选择题
| The economy in the United States is
heavily dependent on aluminum, a material widely used in the construction of
buildings and in making such diverse things as cars, airplanes, and food
containers. In 1979 Americans used over five million tons of new aluminum, and
one and a half million tons of recycled aluminum. Some ninety percent of the
bauxite (矾士) ore from which new aluminum is normally derived had to be imported
to meet the demand. Poorer ores are abundant in the United States, however,
researchers at Purdue University may recently have found a way to obtain
aluminum magnetically from these. Although aluminum is not attracted by ordinary magnets, under special conditions it becomes temporarily "paramagnetic", or very weakly responsive to a magnetic field. This is achieved by immersing ore particles in water to which certain salts have been added and then filtering the ore through steel wool in the presence of a strong magnetic field. It is hoped that this technique will reduce the amount of high-grade aluminum the United States must import. |