What is Sulfide Mining?
Sulfide mining or metal sulfide mining is a term that people in the UP have been hearing a lot lately. It has been used by anti-mining groups and the media, but many people do not understand what it means. Historically, mining has been described by the metal or mineral that was sought. We are used to the terminology of iron, limestone, copper and gold mining.
The ores that are mined for their metal value come in several mineral types. They are mostly oxides, sulfides, carbonates, or silicates. The iron mined at the Tilden and Empire mines are mostly iron oxides like magnetite (Fe3O4) or hematite (Fe2O3). A significant amount of the iron recovered at the Tilden Mine is in the form of an iron carbonate called siderite (FeCO3). Iron also occurs as a sulfide mineral called pyrite (FeS2). This is the “fools gold” we have all heard about. Pyrite is typically not mined as a source of iron because of the abundance of iron oxides available around the world and the added complications of processing the iron sulfide mineral.
Copper, zinc and nickel are some of the metals that also occur in both sulfide and oxide minerals, but unlike iron, the abundant mineral types are sulfides rather that oxides. In order to economically mine the quantity of these metals that are required in society, the sulfide minerals have to be mined.
Much of the copper that was mined in the Copper Country of the UP was unusual in that it occurred as metallic copper. Some very large pieces of copper were found that weighed several hundreds of pounds. This is very unusual as most copper mined today is in the form of sulfide minerals like chalcocite (Cu2S) at the White Pine Mine at Ontonagon.
The mining of metal sulfide minerals is what is being referred to as “sulfide mining” today. It does not refer to the type of mining or processing, only the type of mineral being mined. A common misconception is that sulfur is somehow used in the mining process, which is not true.
The environmental challenge of mining sulfide minerals is that some sulfide minerals will oxidize in the presence of air (oxygen) and then form sulfuric acid when contacted with water. This acid can dissolve minerals and release metal ions such as copper into the water. Acid Rock Drainage (ARD) is the industry’s term for this process.
The ARD chemical reactions have three critical ingredients; water, oxygen and a reactive sulfide mineral (not all sulfide minerals are reactive). Oxygen comes into play when the sulfide mineral comes in contact with the air. When the mineral is in the ground in its natural undisturbed state, it is typically submerged in ground water that prevents contact with oxygen. ARD will not occur in this state. During mining, the dewatering of the ground provides the opportunity for oxygen to contact the sulfide mineral. As the sulfide minerals are transported out of the mine and processed, safe practices have to be followed to manage ARD. The ARD danger to the environment and has to be avoided, contained or neutralized.
In the past, acidic metal-rich waters from mining have damaged the environment when mining companies did not follow safe practices. Today mining companies have to be good stewards of the environment. Our laws are made to ensure this happens. Mining in the past behaved no differently than many other industries, such as chemical manufacturers, automakers and leather tanners, who followed practices that damaged the environment. Those practices are no longer tolerated in today’s society.
The challenge for a company that wants to mine a sulfide ore body is to determine the potential for ARD and then follow the correct mining practices to avoid, contain or neutralize any ARD. An example of good environmental practices is the Flambeau Mine at Ladysmith, Wisconsin. Kennecott was the operator of this open pit copper sulfide mine that operated 140 feet from the Flambeau River in the 1990’s. During the mining operation all of the surface area drainage and pit pumping water went into a treatment plant that successfully purified the water so it could be safely returned to the environment. Upon closure, to avoid ARD, the pit was backfilled with the waste rock that was stripped from the pit along with 30,000 tons of limestone. As the primary ARD safeguard, the ground water level was re-established covering the remaining sulfide minerals in the mine, eliminating the oxidation and preventing ARD for the long term. The limestone was added as an ingredient to neutralize any ARD that formed while the pit was exposed. The limestone will also act as a secondary safeguard to neutralize any possible ARD if the water table somehow receded. The land at the mine site was reclaimed and area monitored for ARD. Areas of surface contamination have been addressed and the ARD threat has been eliminated. The Flambeau Mining Company did not have any violations of its permits in construction, operation and closure. It was closed in 1997, almost 10 years ago.
The Flambeau mine did not concentrate its ore; it shipped the high-grade ore directly to the smelter in Canada. Direct shipping can only be economically done in mines with very high-grade ores. The Flambeau Mine only mined the high-grade ores and avoided having to deal with tailings reclamation. Tailings are generated when low-grade ores are processed in a concentrating plant that separates and rejects a low-value tailing and recovers a high-value concentrate. The downside at Flambeau is that a significant tonnage of lower grade ores was not mined and the copper resource was not fully utilized. That was a significant loss to the local economy as those reserves will not likely be economically mineable in the future. That was a compromise Kennecott had to make because of the regulatory and hostile mining environment in Wisconsin.
Another example of a sulfide mine relying on neutralization to prevent ARD is the White Pine mine near Ontonagon. Copper sulfide ore was mined there for over 40 years and because the host rock contained limestone, no ARD occurred. About 80% of the copper ore mined was chalcocite, which is not normally a reactive sulfide mineral and did not contribute much to ARD. There were minor amounts of bornite, chalcopyrite and pyrite that can generate ARD. The limestone contained in the host rock was very effective in neutralizing ARD. This is an example of a non-acid producing sulfite mine due to the mineralogy of the sulfite mineral and the neutralizing capacity of the host rock. It can be noted that with all of the copper minerals in the ground, there is no elevated copper in the ground water. This again is due to limestone preventing any acid formation thus providing no mechanism for copper ions to enter the ground water.
Sulfide mining has been done in both Michigan and Wisconsin without damaging the environment. Every sulfide mineral deposit is different and requires individual attention, but there are modern mining practices that allow these reserves to be successfully and responsibly mined. The newly adopted metal mine permitting laws in Michigan provide the guidance and enforcement to ensure that responsible mining happens.
A Citizen for Responsible Mining