Skyline modifying milling process at Johnny Mountain

A number of modifications to the 300-ton-per- day mill operating at Skyline Explorations’ (TSE) Johnny Mountain mine in northwestern British Columbia are expected to increase recoveries to 90% and bring about a more favorable ratio of gold produced on site to gold shipped out in concentrates to Japan. According to William Price, Skyline’s new vice-president and chief operating officer, those expectations should be realized after final modifications to the milling process are completed early next year. Although Price estimates that total capital costs will come in under $200,000, he told The Northern Miner the most significant change to the milling process will “cost nothing” and result in a saving of $30 per ton milled.

That’s because Price is overseeing the elimination of the cyanide circuit in its entirety from the milling process. Originally, Skyline planned full cyanidation of ore for the project, but later revamped its plans so that 30% of the gold would be recovered by a gravity circuit, 40% of gold recovered in a copper concentrate containing byproduct silver, and the balance recovered by cyanidation of mill tailings.

But Price explained that a number of problems arose in trying to run the cyanide circuit, particularly because the mill equipment (some of which was purchased used) was in series and if any one component went down it caused the whole system to go down.

“Most of the equipment causing us mechanical problems was in the cyanide end,” Price noted. “Plus the circuit is costly to run because everything has to be flow in.”

Problems also developed because the cyanide circuit had inadequate filtering capacity and would often get plugged up, slowing down milling. In addition, Price said problems with the water supply system wouldn’t allow the former operator to use the gravity circuit at all. A decision to remove the cyanide circuit was made after recent test work on stockpiled ore showed that 90% recoveries could be achieved at 300 tons per day without it.

Price said the company is also replacing two cleaner cells in the flotation circuit with a bank of four smaller cells to give 3-stage cleaning. Modifications are also planned for the concentrate thickener, and a stock tank will be added. Changes to the water supply system are also planned, which Price explained would ensure a better water balance and supply “when and where we need it, and in the quantities we need.”

The equipment necessary for the modifications is on its way to the mine site, and Price said work would begin after the Christmas break and be completed by the end of January. After allowing for tune-up in February, the company is expecting to achieve recoveries in the 90%-range by March. In early December, recoveries were 83% from an average daily throughput of 259 tons per day. Price said he expects production levels will likely remain in the range achieved in the first ten days of December (1,386 oz gold, 2,894 oz silver and 32,676 lbs copper) until the modifications are in place.

Currently about 75% of the gold recovered by milling is going out in concentrates, but Price said he expects that will be lowered to 70% by next spring. The first shipment has already been made to Japan, with a large part of the second load being readied for shipment in Wrangell, Alaska. But the company is now doing further test work to see if it can improve that ratio further in favor of the gravity side, particularly because the process means the company gets paid more for its gold, and it gets paid sooner.

“Our testing so far indicates that about 50% of the gold is liberated, free enough to collect that way in the laboratory. We aren’t expecting to get above 50%, but we think we can approach that figure,” Price said, adding that the company is now looking at ways and means to achieve this, including further fine- tuning or possibly adding equipment such as centrifuges.

On the mining side, Price said the company has been able to get a good handle on the dilution factor for some time now by using a method of drilling and blasting called breasting. After drilling horizontal holes for one short blast, the faces and walls are carefully re-examined and marked up again for greater control, as opposed to drilling uppers which would go right along the whole length of stope making any miscalculation a “mining fait accompli.”

Price said mill head grades have been going up steadily every month, reaching 0.61 oz gold in mid- December. “I think that’s representative of what we’re going to see for this year anyway,” he added.

Looking ahead to 1989, Price said the company will begin the year with a complete mine plan, mine budget and a new chief engineer, with a permanent mill superintendent expected shortly.