Ziniflex Century Ultrafine Milling Circuit

The Ziniflex Century lead–zinc deposit is a massive fine-grained ore body located in the northwestern region of Queensland, Australia. Minable reserves are approximately 100 Mt at an average grade of 1.7% lead and 12.5% zinc. The flotation plant has a design throughput of 5 Mtpy, and the first ore was introduced to the plant in November 1999.

The deposit was originally identified by Conzinc Riotinto of Australia (CRA) in 1990, who conducted the definitive feasibility study during 1994 and 1995. Rio Tinto Zinc Corporation (RTZ) sold the project to Pasminco after the RTZ merger with CRA during 1997.

The main process problem with the Century ore is that the sphalerite, which is the main zinc mineral, is very finely interspersed with silica (Hookham and Sutherland 1999). Ultrafine grinding to a P ₈₀ of 6.5 Pm followed by flotation was considered the most cost-effective and practical solution. Fifteen Metso 355-kW SMDs were installed at Century to produce the desired product size with an additional six units in a zinc regrind duty .

As mentioned previously, small media, smaller than 5 mm, is generally more energy efficient than coarser media when the material feed size is greater than 100 Pm. This accounts for the relatively low work input values to mill to a P80 of 6.5 Pm. Century uses a sand media with a mean size around 2 mm.

Individual SMD performance is monitored constantly, and mass flow rates and the number of SMDs operating is adjusted continually to allow for the varying flotation mass pull. This minimizes over- and undergrinding in the zinc circuit. One limitation of all fine and ultrafine grinding circuits is the lack of a reliable industrial on-line particle-size analyzer capable of measuring sub-20-Pm particles.

Overall, it was found that the laboratory and pilot studies conducted before the construction of the final mineral processing plant provided accurate plant sizing data to the engineering company charged with designing the plant and Metso process engineers. This was aided by the quality of the original test samples, a combined effect of the quantity of samples mined and processed.