Now [NFC Africa Mining's Chambishi mine] employs 2,000 people...But in spite of soaring copper prices, some miners are paid only $100 a month. They complain of exploitation - "slave labour" in the words of one. The mine's deputy chief executive, Xu Ruiyong, speaks of "local friends" and says there is a cultural gap. "Maybe we think it's normal, but other people think it's too harsh," he adds.