Vintage metallurgy now “chemically correct”

With a wary eye on the environmentalists, quite a few gold mining companies are making a big to-do about recovering gold without cyanide. In this day and age, being “chemically correct” is highly desirable. It might even determine whether a company goes into production or not. So, companies are wholly within their rights to trumpet that they don’t use the sadly maligned reagent.

But, if there is also the inference that the technology is new, then that is pushing the case a bit too far. A few wrinkles have been added in recent years; there is improved equipment for some mines, but otherwise the recovery of gold by gravity followed by flotation is vintage metallurgy. Besides, if coarse gold is not recovered by gravity, and regardless of whether the mill employs flotation or cyanidation processes, it is almost certain the coarse gold will be irretrievably lost.

For example, if the gold mill uses flotation, the froth may not be tough enough to hold a heavy gold particle for a sufficient length of time. The carrier-bubble punctures, the gold particle sinks and it passes into the tailings stream.

The phenomenon was noted at a large-scale, tailings retreatment project in northern Ontario some years ago and is believed to have contributed in some measure to that project’s failure.

If gold recovery is by cyanidation, 12-30 hours is needed to dissolve the metal even though it is finely divided. So, if there is any coarse gold present, only a little will be dissolved and the rest will pass into the tailings stream. This is how gold found its way into the tailings at the failed project referred to above.

In either type of mill, large particles of high specific gravity gold will tend to drop out of the mill pulp at an early stage and lodge in any crevices they can find. The operating company recovers some of the accumulated treasure trove when ball mills are relined, when pump boxes are cleaned out and so on. Most, however, will not be recovered until the mill is permanently closed and the equipment dismantled in preparation for repair and resale. Dismantling the mill was often left to salvage-contractors in an earlier era. If anecdotal evidence is anything to go by, many small fortunes were made in Ontario and Quebec. The contractors literally “cleaned up.”

Therefore, gold mills incorporate gravity circuits whenever coarse gold is present in the ore. Gravity separators are generally located alongside the ball mills. It is there that the metal is first separated from its enclosing rock; it is there that it becomes free gold.

Until about 20 years ago, a common gravity circuit consisted of one or more pulsating jigs and an amalgamating barrel (a small size ball mill, 4-6 ft. long and 2-3 ft. in diameter).

A low-grade concentrate was produced by the jigs, but its low grade (1-4% gold) was not of any great concern. The metal was readily extracted by mercury and the residue left behind in the amalgamating barrel was returned to the mill for treatment along with the bulk of the ore.

The mercury/gold amalgam was retorted, the mercury boiled off, the vapor condensed and the liquid metal re-used. The gold remaining in the retort was melted and cast into bars.

It is no longer practical to use mercury for environmental reasons. Nor is it practical to smelt the low-grade concentrates that a jig produces. The concentrate must be further upgraded and this is generally done on shaking tables. Sometimes jigs are omitted entire-

ly as a lot of operators find them too finicky. Spirals are used instead. Some gold mills have turned to specialized centrifugal concentrators. These machines were originally developed for recovering the fine gold of low-grade alluvial deposits.

Whatever technique is employed, the concentrate will now grade up to 50% gold. All that has to be done is to slag off the contaminating waste rock with fluxes and cast the gold into bars.

The Snip mine in northwestern British Columbia, owned 60% by Cominco (TSE) and 40% by Prime Resources (VSE), is an example of current gravity/flotation practice. The mill first turned over in late January, 1991: 135,200 tons grading 0.89 oz. per ton were treated that year. Bullion derived from gravity concentrates was responsible for 25% of total gold production. Copper flotation concentrate assaying 12.8 oz. gold per ton was responsible for the balance.