Optimisation of paste thickening at the Yara Siilinjärvi plant

Abstract

The Yara Siilinjarvi plant is located in central Finland. The site consists of a mine, two sulphuric acid plants, one phosphoric acid plant, one nitric acid plant and one NPK fertiliser plant. The Siilinjarvi mine produces about 1 Mt of apatite concentrate, which creates approximately 10 Mt of tailings per year. At the beginning of 2017, tailings handling in Yara Siilinjarvi was brought into a new era, when the conventional tailings storage facility was replaced with a new tailings treatment plant. The new high-density tailings disposal system with two deep cone-type paste thickeners increased the percentage of solids in the slurry from 45–48% to 66–69%, making it possible to deposit as paste. This extended the lifetime of the current tailings storage facility by 15 years – from 2020 until 2035. The rheology of the tailings in Siilinjarvi is unique, due to the minerology of the slurry. In particular, the high content of coarse mica makes thickener operation challenging and makes high-density pumping sensitive to disturbances. Therefore, the importance of process control cannot be underestimated. An advanced control system is needed to maintain process stability and the desired operation point in varying running situations. This paper presents the results from applying a multivariable, optimising control strategy to two paste thickeners at the Yara Siilinjarvi plant. The presented solution is based on a separate advanced control tools (ACT) platform, and it can be cost-effectively utilised with any customer distributed control system/ programmable logic controller (DCS/PLC) system available. With the ready-made application package, implementation is straightforward, and only a minimal amount of system-specific tailoring is needed. The ACT system in Yara is highly integrated with the plant DCS, so that the operators have full access to the user interface, alarms, interlockings, history data and documentation through the DCS displays. Long-term follow-up shows that with the optimising control, the Yara paste thickeners can consistently run with underflow solids content of 66–68% to average beach slope angles of 3.5° (6.1%) in the tailings disposal area. Additionally, this control system facilitates the minimum use of flocculant, which means 10–20% savings in the flocculant costs. Robust thickener operation also leads to trouble-free high-density pumping and high availability of the paste plant.