The average mill heads at Nanisivik for 1988 were 10% zinc and 0.4% lead, says Mill Superintendent John Goyman. He adds that the zinc grade varies greatly now that mining is being focused on the satellite zones. Lead head grades have fallen off considerably over the past 18 months, which has posed some problems in establishing a good concentrate grade.
Sphalerite and galena are the ore minerals, with 40 g silver per tonne reporting mostly with the zinc concentrate (not with the lead, as is usually the case). The metal is in a solid solution in the sphalerite, and all attempts to separate it out have proven unsuccessful. As a result, the silver must be shipped out with the zinc concentrate.
Ninety per cent of the waste consists of pyrite; the rest is dolomite.
“Ore is ground at about 55% minus 200 mesh,” says Goyman. “That is where we get our best combination of concentrate grade and recovery.”
The Nanisivik rod mill is a 2.9x 3.6-m Hardinge with Wabi Ni-Hard liners; it uses 9-cm steel rods. The ball mill is a 3.2×3.6-m Hardinge with a Skega rubber lining, which uses 5-cm steel balls. The rod mill has a 500-hp drive; the ball mill a 700-hp drive. A set of the Ni-Hard liners is good for 600,000 tonnes of ore, says Goyman.
Grinding is accomplished in a closed circuit consisting of a ball mill, unit cells and cyclone.
The cyclone overflow passes through one bank of lead roughers (six Denver 30s) and then through a set of Denver mini-cells which have three additional cleaning stages. The lead scavenger product is then run through a conditioner cell (where it is treated with copper sulphate to activate the zinc) prior to entering the zinc circuit.
Both circuits have a very high ph (12.25), but the lead won’t float at any less a ph. So far, no one has been able to explain why such a high ph is needed, Goyman says, but it works.
The final zinc concentrate grade runs a consistent 56%, but the lead concentrate grade varies with the head grade, Goyman says. Zinc recovery is 96%; and recovery of lead is between 84% and 85%, but only if the heads are 1.5%. However, lead recovery is lower during those times when emphasis is placed on maintaining high concentrate grade.
The average reagent consumption is as follows: Lime ….. 2,400 g per tonne Copper sulphates ….. 625 g/t Xanthate …………. 180 g/t Frother ………….. 5 g/t Mill rods ………… 490 g/t Mill balls ……….. 520 g/t
“We are experimenting with a column flotation cell in our lead circuit in an effort to improve concentrate grade,” Goyman says, “although it is not our intention to replace conventional cells. It is not yet known what the results will be.”
To monitor the milling process, eight samples are taken from different points in the mill every hour and tested in an Outokumpu X-Met analyser.
A 3-m, 8-disc Eimco disc filter is used to dewater the zinc concentrate, while the lead concentrate is dewatered in a Denver 2-m, 4-disc filter. Zinc concentrate goes to a 15-m-diameter thickener and the lead goes to a 7.25-m thickener, both with Dorr-Oliver mechanisms.
Two Koppers dryers are used to dry the zinc and lead concentrates. They were designed without burners so that, instead, they utilize the hot exhaust gases from the diesel generators used to supply electrical power to the mine and town. Two 20-ton trucks haul the concentrate to the storage building, on the shores of Strathcona Sound.
About 80% of the 85-tonne-per- hour mill feed rate is pumped to the tailings pond. The primary tailing pump is a Geho ZPM 600 positive displacement pump, which is backed up by a Mitsibishi Mars positive displacement pump.
Tailings are pumped to 25-m-deep West Twin Lake, where the ph level reaches 11.6 in the winter and drops to 11.2 in the summer, says Goyman. A 5,000-m Sclaircor tailing line is used: the first 1,200 m being rubber- lined steel pipe; the remainder, Du Pont Sclairpipe with 75 mm of styrofoam insulation around it, covered with a steel outer shell. The line is heat-traced to prevent freezing, and sensors at each installation notify the control room at the power house if the temperature becomes dangerously low.
The tailing line generally operates at a pressure of 29 bar and the static head is 130 m. Territorial limits set for metal content in discharge fr om West Twin Lake are strict: 0.1 mg lead per L, 0.2 mg zinc per L and 0.005 mg cadmium per L. Almost too strict, says Goyman.
“The more tailings we put in the lake, the less water volume there is, and the amount of metal in the lake is definitely increasing. We’re starting to have problems with our cadmium levels, for example.
“To rectify this problem, we’re looking at forms of effluent treatment.”
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