The only mine unit owned by Outokumpu, the chrome mine in Kemi, Northern Finland, is busy making preparations to double its production, set to begin next year. In co-operation with Metso, Outokumpu will increase the number of underground feeders and optimise the durability of wear parts in the massive primary gyratory crusher. Up until now, the entire production of the chrome mine, totalling about 1.3 million t/a, has been transported to the Tornio ferrochrome plant for processing and on to the Tornio stainless steel plant. The increase in production, which will begin in autumn 2012, is intended also to enable Outokumpu to sell ferrochrome to companies outside the Group. The production target for 2013 is 2.7 million t of ore, and of the ferrochrome produced from the ore, nearly a half (> 200 000 t) is to be sold outside.
About 500 m underground, the majority of the work involved in increasing the capacity, the F3 project, has already been completed. Doubling the production is no problem for the Superior® primary gyratory crusher (Fig. 1), delivered to the mine by Metso in 2002, which can digest more than 2700 t/h of feed material if necessary. At the moment, the primary gyratory crusher can be fed by three different vibrating feeders (Fig. 2). The crusher currently runs for less than one-fifth of the production time. The increase in ore production will increase the effective crushing time to about one-third of the production time.
A lot of work has been done for ore storage after the primary crusher. The two currently used storage silos are now accompanied by two new 8000-t silos. The number of Metso feeders transporting the ore from the primary crusher has also been doubled.
Chrome ore has been a challenge to the primary gyratory crusher’s wear parts. The wear is contributed to by the pulsating feed, which rarely fills the entire crushing chamber. There are also great differences in ore type, since brittle feed material prone to chalking is occasionally replaced by hard ore that requires a lot of crushing power before it is crushed with a bang.
The Kemi mine and Metso are currently carrying out a development project to increase the service life of the mantle and outer concave and thus minimise downtime due to replacement. While the modular feed hopper enables quick replacement of the mantle, replacing the outer concave can easily take more than a week. The co-operation between the Kemi mine and Metso’s wear part team in Tampere aims to extend the service life of the parts to at least 4 million crushed t. Durability is sought by, for example, optimising the chamber structure and selecting the right materials for the outer concave.
Mika Saari, Foreman of ore line technicians working in the underground maintenance of the Kemi mine, leads a small but experienced team of technicians (Fig. 3 and Fig. 4). The tidy, modern office spaces are located close to the action, at a depth of 500 m. “Co-operation with Metso has been smooth. Increasing our own crushing expertise takes time, so all know-how shared by the manufacturer is very welcome to us,” Mika Saari says. “We can also use Metso’s professionals for servicing when necessary, like we last did in July for the replacement of the crusher’s wear parts. Our work is also facilitated by product development, like the addition of maintenance hatches to the feeders,” he continues.
According to Mika Saari’s calculations, the Superior® primary gyratory crusher has so far crushed more than 6 million t of ore in total. “The factory polishing marks still show on the bottom thrust bearing, so we believe that the bearing will last the entire remaining service life of the crusher, which is about ten years and 30 million t. Metso’s feeders have been equally reliable,” Saari says. After the next 30 million t of ore, it will be time to deepen the mine.