Zimbabwe has imposed a ban on the export of unprocessed lithium, according to a government notice seen by Reuters on Tuesday, in a bid to stop artisanal miners who the government says are digging up and taking the mineral across borders. The southern African country holds some of the world's largest reserves of hard rock lithium, a vital mineral in the production of clean energy technologies. Chinese giants Zhejiang Huayou Cobalt, Sinomine Resource Group and Chengxin Lithium Group have over the past year acquired lithium mines and projects worth a combined $678 million in Zimbabwe and are at various stages…