Cobalt Mining Legacy

Mining and Milling in Cobalt

Geology of the Silver Veins

Some understanding of the geology of the silver deposits in the Cobalt area is important to understanding the sources of contamination in the area.

The silver occurred in veins which varied from a few millimetres to over 1 m in width. Some of the richest veins in the area were on the west and east sides of Cobalt Lake. Many veins were exposed at surface, and were heavily weathered.

The ore in Cobalt is very complex, with a unique group of minerals that occur in few other places. The bulk of the veins are composed of calcite, dolomite, quartz and chlorite. Silver normally occurred is pure, or “native” silver. The veins also contain many different arsenide, and sulfarsenide minerals as well as some sulphide minerals. Arsenide minerals are minerals in which a metal or metals are bound to arsenic. Sulfarsenide minerals also contain some sulphur.

The ore contained large amounts of arsenic, with the results of chemical analyses giving arsenic concentrations in ore ranging from about 32% to almost 65%. In addition to silver, the most common metals in the ore are cobalt and nickel, and some also contain mercury.

When exposed to water and oxygen, the arsenide or sulpharsenide minerals can be chemically changed, or altered. The result of the formation of what are known as secondary minerals – minerals formed as a result of the chemical alteration of other minerals.

One of the most common secondary minerals in Cobalt is erythrite, also known as cobalt bloom. Erythrite is a very distinctive light pink to purple in colour. Early prospectors used it to help find veins, and early descriptions of the geology of the area report considerable quantities of erythrite. Since erythrite can itself be altered, releasing arsenic to the environment, the fact that there was a lot of erythrite around in Cobalt in the early days suggests that even then, the water around Cobalt contained arsenic.

Milling Processes Used in Cobalt

Milling is the name given to the process of grinding ore and recovering the metals of value from the ore. An understanding of the milling methods used in Cobalt helps in understanding the nature of the tailings and other mine wastes in the area.

The complexity and unique character of the silver ore from Cobalt presented many challenges to metallurgists. At first, there was no way to mill the ore in Cobalt. From 1904 to 1906 ore was hand sorted and high grade ore was shipped elsewhere, often to the United States, for processing. The mining still produced waste rock, which is rock that has no value but needs to be removed to be able to mine the ore, but there were no tailings from these early operations, since tailings are a by-product of milling.

Shipping the ore elsewhere for milling meant losing money, so mine owners looked for ways to process the ore in Cobalt. In 1907, the first mills were set up in Cobalt. In these mills, ore was first crushed, then pulverized in stamp mills. In the first mills, gravity concentration was then used to concentrate the silver, producing a product known as silver concentrate. The concentrates were shipped elsewhere for refining. The waste product of this milling was powdered rock, called tailings.

The gravity concentration process was an improvement over shipping the ore out of Cobalt for processing, but still resulted in the loss of a lot of silver to the tailings. And it still meant shipping the concentrates elsewhere to complete the process of recovering the silver.

In 1909 the first cyanide mill was put into operation in Cobalt. In the cyanide process ground ore was mixed with a solution of potassium cyanide which dissolved the silver. Powdered aluminum was then added, causing the silver to precipitate. The precipitate was melted, yielding a silver bullion.

Several cyanide mills operated in the Cobalt area, and the tailings from these mills contained cyanide as well as arsenic, nickel and cobalt.

The cyanide process, much refined, is still used today at many mills for the processing of gold ores.

During World War I, many mills switched to flotation separation. In this process the ground ore was mixed with creosote, pine oil and coal tar. The ground ore became emulsified. The emulsified ore was agitated in water in flotation cells, with air bubbling through the cell. This process caused the ore minerals to attach to air bubbles and float to the surface of the cells. Other minerals sank to the bottom. The ore concentrate was recovered from the cell and sent for further processing to recover the silver. Waste minerals were disposed of as tailings.

The highest grade ores from Cobalt presented unique problems for milling, and initial recovery rates were low. In 1911 the Nipissing high grade mill (NHGM) was brought into operation. In this mill high grade ore was ground in mercury and potassium cyanide. After grinding, 98% of the silver was amalgamated with mercury. The amalgam was then refined in furnaces to produce a final silver bullion. Mercury was also used at other mills in the area, including the Northern Customs Mill and the Buffalo Mill.

The tailings from high grade mills like the Nipissing mill contain mercury in addition to arsenic, nickel and cobalt.

Those interested in learning more about the environmental legacy of Cobalt can go to following pages: