Pilot cooling tests conducted for high-copper converter matte ingots at Polar Division of PJSC MMC Norilsk Nickel

This paper describes the test methods that were applied to three ingots of high-copper converter matte with the weight ratio of Cu:Ni ~ 2.3 at Polar Division of PJSC MMC Norilsk Nickel to determine their cooling rate. The obtained results are also discussed. A test heat was done in the converter of the Nadezhdinski Metallurgical Plant, during which the following compositions were closely monitored: that of middlings; that of the finished matte that was discharged at 1,255–1,260 оС and poured in molds at 1,200 and 1,140 оС; and that of metallized matte that was intentionally overheated to 1,285–1,290 оС and poured at 1,200 оС. The cooling time was 82 and 93 hours (for capped ingots). The temperature was monitored with the help of six chromel-alumel thermocouples located at the level of 1,350, 850 and 350 mm from the bottom; three of them in the centre and the other three on a side. The ingots were crushed at the Nickel Plant, and a ~300 kg representative sample with the size of 3 mm was taken from each ingot for flotation tests. Spot samples were taken from the temperature monitoring areas for structural characterization. Based on experimental data, functional dependencies were derived with R2 0.99 that describe the obtained cooling curves. It is shown that due to the use of insulation cap in the structure defining temperature range of 1,150–750 оС, the cooling rate of the ingot top can be considerably decreased (approximately by three times). It is equal to 28–29 оС/h. In the vertically central part of all test ingots — i. e. ~850 mm from the bottom both in the centre and on a side, the cooling rates vary in the range of ~9–10.5 оС/h; in lower monitoring points — i. e. 350 mm from the bottom, they are equal to 24–33 оС/h. The cooling rate of the overheated matte at the top drops to 67 versus 87 оС/h for the uncapped ingot, whereas in the centre and at the bottom it almost remains the same.

Effect of copper-nickel matte ingot cooling time on its floatation separation selectivity indicators

A criterion used to evaluate the efficiency of converter matte foam separation into nickel and copper concentrates is a selectivity index based on the total recoveries of metals into target concentrates that in turn defines their cumulative impurities (secondary metals) content. In addition to various factors (meeting density and reagent flow charts, comminution parameters, etc.), the time of preceding cooling of ingots is also known to have a substantial effect on the process of converter matte separation at commercial scale. Laboratory studies on selective separation were made to evaluate the influence of converter matte crystallization conditions at constant comminution and floatation parameters. Commercial converter matte ingots produced at different cooling rates were ground and floated in the closed circuit under laboratory conditions according to the existing floatation flowsheet. The lab studies allowed to exclude the multifactor nature of the system and to examine the commercial converter separation process only from the viewpoint of converter matte melt cooling rate since the other factors were kept constant during the laboratory tests. The temperature field in the body of the converter matte ingot was measured during its cooling in the conditions of the current production – this is reflected in the chemical and phase composition of various ingot sections. The temperature of the ingot, due to its massiveness, varies considerably throughout the material volume. A small change in the ingot surface temperature can be accompanied by significant changes in the temperature in its body. The measurement results showed that the temperature gradient from the center to the periphery of the ingot exceeds400 °C. In this regard, reducing the time of converter matte cooling can lead to significant violations of the cooling mode in the central zones of the ingot. In accordance with the optical mineralogical analysis of samples, the longer was the ingot cooling time, the higher was its decrystallization implying the formation of coarse-particle structures of copper and nickel sulfides with sharp interface boundaries. The chemical analysis revealed that the highest possible selectivity index of converter matte copper and nickel separation with resulting copper and nickel sulfide concentrates, respectively, is reached after 72 h of cooling for converter matte ingots from the smelting shop of the Nadezhdinsky Metallurgical Plant.