New treatment for complex ores gets field test in New Brunswick

The treatment of refractory gold ores has spurted ahead in the last 10 years with plants now operating in Nevada, California and Utah. Canada has not been left out of the picture — the first autoclave plant recently came on stream at the Campbell mine of Placer Dome at Red Lake, Ont.

But that is for gold ores. Little known except to the specialist, hydro-metallurgy and autoclaving have been used for many years to extract nickel and copper-nickel and, as with refractory gold ores, most of the technology was developed here in Canada. Now, a new process is coming on to the scene, CANMET’s “ferric chloride leach,” and its field will be the treatment of complex copper-zinc-lead ores. (CANMET, or Canada Centre for Mineral and Energy Technology, is a research establishment with a $100-million-plus budget funded essentially by the federal government).

These base metal ores are relatively commonplace but in some regions of the world, the minerals are fine-grained and intimately bonded to the extent that they are difficult to process, in a word refractory. A separation of their several components is practical but only at the cost of losing valuable mineral in the tailings.

Such ores are mined in New Brunswick. Similar multimillion-ton deposits have been delineated in Australia and are awaiting a solution to their difficult metallurgy.

CANMET’s chloride leach may be the answer.

The elements of chloride leaching had been investigated in Europe, Australia and the U.S. over a number of years. By the early 1980s, the possibility of developing a commercial process was recognized by a number of metallurgists, CANMET’s among them.

CANMET embarked on a program to develop the process and its 10 years of research and $10-million investment will soon culminate in the construction of a 550-ton-per-day pilot plant in New Brunswick.

In its essentials, the ore will be milled conventionally to produce a bulk flotation concentrate, a concentrate containing all the valuable copper, zinc and lead minerals. Most of the waste rock will be eliminated at this preliminary stage. No attempt will be made to separate the individual metal sulphides by secondary flotation as is done in today’s mills. This is where serious metal losses occur and where the leaching process consequently comes into its own.

The bulk concentrate will be agitated with a ferric chloride solution at close to 100 degrees C and at atmospheric pressure. Leaching will take about 4-4.5 hours. All the valuable metals will be taken into solution as chlorides. The remaining solids consist of iron pyrite (pyrite is not affected by the leach

solution to any extent), elemental sulphur and inert rock particles. The various metals will be recovered individually from the solution by a series of established processes, the ferric chloride of the leach solution regenerated and then cycled for reuse.

There will be no sulphur dioxide emissions and the final residue, purged of its toxic heavy metals, will be accumulated underwater at one of the area’s several completed open pits. This is an environmentally benign metallurgical system with metal recoveries in the 90%-plus range.

The process, of course, has its detractors. There is not a single area of human endeavor without these accompaniments; and something that is new draws the doomsayers like a magnet. Even Napoleon Bonaparte ridiculed the idea that ships made of iron would float.

NovaGold Resources (TSE) financially assisted CANMET in researching the leaching process. The research was adapted to meet NovaGold’s particular needs, the production of a silver-rich copper metal rather than the copper-zinc-lead of the standard process. The company has now concluded a licensing agreement with CANMET.

The pilot plant is to be built at NovaGold’s Murray Brook mine and construction is expected to be under way within the coming 12 months. Costs are subject to revision and are currently estimated at $45 million. NovaGold is already producing gold at the property using a novel cyaniding process. (There are only two other housed, vat leaching plants worldwide.)