Boliden’s smelter in Kokkola established zinc losses in their circuit, which were identified to originate from the silicate minerals in the ore mix. Garpenberg’s concentrate, a product of Boliden’s Cu-Pb-Zn mine in central Sweden, was identified to be the root of silicates in Kokkola. Garpenberg’s concentrator employs a froth flotation circuit in several steps to yield separate copper, lead, and zinc concentrates. The aim of the project is to gain insight in the mechanisms concerning the recovery of silicates and ways to depress their grade in the zinc concentrates by means of adding various depressant reagents, froth washing, and other methods. It was found that dextrin, the presently used depressant, performs sub-optimally, and other reagents exist that outperform it in laboratory scale flotation tests. Furthermore, strong indications were found that there is a detrimental effect in particle size distribution; finely ground silicate particles are recovered through aggregation with sphalerite or entrained with the aqueous bubble flow, and larger joined particles will carry silicates with them if they are recovered. Moreover, a collateral effect is that a large fraction of cleaner tailing zinc is present in the +90 µm coarse fraction, ultimately being detrimental to the circuit recovery.
