问题 问答题 简答题

简述石墨焙烧法与铝液焙烧法相比的优缺点?

答案

参考答案:

优点:

1.可以避免铝液直接倒入冷态熔池,对炭块和扎缝造成热冲击;

2.石墨电阻比铝液大,预热时间缩短;

3.可将电流引向熔池侧部,使侧部扎缝也得到加热;

4.启动初期只有电解质,可以填补阴极裂缝及缺陷。

缺点:容易造成电流分布不均。

单项选择题

It is clear that some chemicals can damage the health of animals and humans. (1) , this is not the only problem that (2) be caused by the careless use of chemicals. Chemicals can also (3) the ecological balance of the environment. If the ecological balance is disturbed, the results can be (4) serious.The (5) of DDT illustrates the problem. DDT, a chemical which kills insects, at first seemed to be a perfect answer (6) many problems. It would control insects that caused dangerous diseases, (7) insects that caused billions of dollars of damage to crops every year. Governments permitted and even (8) the use of DDT. Farmers in many countries (9) to spray it on their crops. The (10) results were good. Damage to crops (11) down, and profits went up. However, the chemical had effects which the scientists didn’t see in (12) . First, it also killed insects which were the (13) enemies of the harmful insects and which were therefore beneficial to farmers. Second, and perhaps worse, DDT did not kill (14) harmful insect. A few insects had natural resistance to the chemical. They (15) and reproduced in large numbers. In a few years there were large numbers of insects which were not (16) by DDT, and there were (17) insects which Could act as natural (18) on these new "super-insects". Finally, it became clear that DDT was not solving the insect problem. In fact, it was making the problem worse. It (19) became necessary to find a second (20) for the effects of the first.

第(8)空应选择()

A.asked

B.requested

C.declared

D.encouraged

判断题