As the use of coal underground mining face of electrical equipment and control equipment, must be flameproof or intrinsically safe type equipment.

但是由于在煤矿井下采掘工作面使用的电气设备和控制设备，必须是隔爆型或本质安全型设备。

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In the early days of underground coal mining, pit canaries often had short but valuable lives.

早期地下采煤的时候，矿井中的金丝雀常常拥有短暂而有价值的一生。

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Mobility between surface mining, underground mining, coal or metal mining sectors may be limited by differences in production technologies and licensing requirements.

表面开采，地下开采，煤或金属采矿行业之间的流动性可能会因生产技术和许可要求的差异而受限制。

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And while it's true that's shrinking as a percentage of Alpha's business, because we're exporting a lot more coal this year than we have in the past, our domestic steel companies are scrambling to get coal since metallurgical coal comes mostly from underground mining.

FORBES: Magazine Article

Coal mining accounts for 90% of sales at Joy's underground division and 33% at its surface-mining unit (copper and other ores account for the rest).

FORBES: The Big Dig

Louis-based Patriot Coal agreed to phase out its mountaintop excavationsand redirect its efforts back to underground mining.

FORBES: A Turning Point for Mountaintop Removal Coal?