hard coal造句怎么写
- 造句
- 关注:2.18W次
The first caving roof sections in hard coal layers can effectively reduce the pressure during the course of caving and guarantee safe production and efficiency.
anthracite; hard coal
hard coal; anthracite
Finally great depth of burial, long time and higher temperature may metamorphose the lignite into soft coal or bituminous coal and in extreme cases, to hard coal or anthracite.
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