Yara International ASA is a Norwegian fertilizer company. Its largest business area is the production of nitrogen fertilizer. Yara Siilinjärvi site in the Eastern Province of Finland consists of a phosphate mine, two sulphuric acid plants, one phosphoric acid plant, one nitric acid plant and one NPK-fertilizer plant. The mine together with the phosphoric acid plant supplies the phosphorus raw material to the NPK fertilizer plants and feed phosphate plant in Finland. Annually the mine produces about one million tonnes of apatite concentrate, and about ten million tons of tailings.
Background to the project
In 2015, Yara´s tailings area was filling up and was close to its end of life capacity. Teija Kankaanpää, Head of Siilinjärvi Mine had three options: build a new tailings storage facility, raise the dams, or invest in new technology that would allow the use of the old tailings storage facility for years to come. Not wanting to go the route of creating a new tailings pond and expanding the footprint of the facility, Teija decided to turn to a trusted partner.
Thickening of the Yara Siilinjärvi tailings was first studied in the 1980s. At the time, the technology to process tailings and achieve a two-degree or more beach angle at the tailings discharge was not yet available. The challenge was the unique properties of the tailings material. The material is coarse, but also contains a high proportion of mica and a fine fraction, which is difficult to aggregate with the coarse particles using flocculation. The material settles and consolidates rapidly to form a highly structured material at a high density and yield stress. This creates problems in mixing tanks, thickeners, pumps and pipelines, and causes blockages if the material segregates and consolidates.
In 2010, Outotec worked with Yara for a thickened tailings project at the Siilinjärvi site. The work included initial test work and sizing for two 35-meter high compression thickeners. Based on the test work, Yara commissioned Outotec to build a demonstration plant with a 14-meter thickener. The plant was in operation in 2013-2014. During its operation the design criteria for the full-scale thickener plant and discharge system was established. In 2015, Yara awarded Outotec with the €40 million contract for the full-scale paste thickening plant as an Engineering, Procurement and Construction (EPC) project.
“There are few companies in the world that can supply this kind of technology that we were looking for, and who had also track record in supplying this kind of plants,” said Teija. “We chose Outotec because they have good technology, and they were willing to offer us an EPC contract.”
In 2017, the paste plant with two 30-meter paste thickeners went through water testing in freezing minus 25 degrees Celsius (-13 degrees Fahrenheit) weather. The performance test passed within one month of the start-up, and the plant has been in operation since February 2017.