Thickener modernization improves process performance in Russia

Nornickel is Russia’s leading metals and mining company, and the world’s largest high-grade nickel and palladium producer. The company’s Talnakh concentrator processes rich and cuprous ores from the Oktyabrsky and Talnakh deposits to produce nickel, copper, and pyrrhotite concentrates.

Concentration processes in general consume vast amounts of water. Closing water cycles and applying circulation water supply systems is one of the approaches being taken by concentrator plants to make more efficient use of water resources and prevent environmental pollution. Purifying water from contaminants to the maximum allowed concentration (MAC) level as determined by the local laws for effluent waters represents a significant cost for concentrator plants and increases production costs. The high capital cost of wastewater treatment up to MAC level, unacceptable contamination of the environment, and less stringent requirements for circulation waters make water circulation systems a necessary investment.

The Talnakh concentrator has a closed water circuit. Since 2017, water from the tailings pond, purified mine water, and overflows from the plant’s thickeners are used in the process. The circulation water flows are combined in the storage pond, which is also used for preliminary water treatment, where residual concentrations of flotation reagents are treated using activated carbon and conditioning. The main monitored parameters for circulation waters are рН, concentration of ions affecting flotation efficiency, residual concentration of flotation reagents, and the quantity of oil products and suspended substances.

Prior to modernization, the concentration technology included thickening of ground crude ore, copper circuit tailings, and the concentrator’s end products. Fluctuations in the ore composition supplied to the concentrator, an increase in processing volumes, and deterioration of raw material properties together resulted in solids increase in the thickener overflows. Because the thickening area had been operating under confined conditions for a long period of time, a large volume of material had accumulated in the storage pond – more than 65% of which consisted of fine-dispersed material of below 10 µm with approximate sulfides content of 30%, including superficial oxidized sulfides.

A project to modernize and revamp the operations at Talnakh has been ongoing since the end of 2014. The initial aim was to increase the crude ore processing capacity by 30% by 2016, with the subsequent target to increase by a further 80% by 2023. During the modernization, it was decided to retrofit seven radial thickeners with Outotec High Rate Thickener technology. The key design features of Outotec HRT thickeners are:

• Feedwell embedded at the level of the diluted layer
• Reflecting cone
• Feed de-aeration and automatic feed dilution
• Automatic monitoring and control of flocculant supply and thickener bed level
• Rakes with low hydraulic friction
• Hydraulic drive unit

The thickening process in the high-rate thickeners is based on efficient flocculation – formation of loose flaky aggregates (floccules) from small particles of the dispersion phase that are suspended in a fluid medium. Automatic feed dilution is based on the natural difference between the hydraulic pressure in the feedwell and the thickener itself, which results in partial entry of overflow to the feedwell, where the source slurry is diluted, mixed, and flocculated.

In the upper zone of the Outotec feedwell, the slurry is mixed with water for automatic feed dilution and contact with the flocculant solution (Figure 3). This ensures aggregation of particles of all sizes before the slurry mix is fed to a formed bed. The diluted bed functions as a filter and generator of floccules that capture any fine particles not flocculated in the feedwell. Thickener operation can be controlled by changing the source slurry feed rate, the flocculant dosing, and the rate of consolidated product discharge.

Reduced solids content in thickener overflow

Modernization with Outotec HRT technology together with the implementation of automatic flocculant preparation and dosage units ensured gains in productivity for the Talnakh concentrator while meeting solids content norms in thickener overflows, as illustrated in Figure 5. The solids content in thickener overflows dropped significantly during the modernization of the thickening area between January and October of 2017.

Increased capacity with a reduced thickener footprint

After the modernization, despite the increased concentrator capacity, the area required for the thickeners was reduced by 1275 m2. This is because the capacity of the modernized thickeners is between three and ten times greater than that of regular static thickeners. Modernization eliminated the risk of a shortfall in the production capacity of the thickening section and an overall decrease in concentrator efficiency.

The next stage of the expansion project will further increase the concentrator capacity. After this stage is complete, the concentrator will be capable of processing approximately 18 million tons of ore per year, including a mixture of impregnated and cupriferous ores from the Oktyabrsky and Talnakh deposits in one stream. This stage will also require further modernization of the thickening area.