When a mining company wants a specific mineral, it has to remove rock to get at it. Only a small amount of rock – ore – will contain the mineral that the company wants. With open pit or open cast mining, the soil, vegetation, and rock above the ore body, called the overburden, must also be removed. Any other rock that is removed that does not contain "economic amounts" of mineral is called waste rock. Mining companies put this waste rock in piles or "dumps" (often measuring several square kilometres). In many cases it poses serious environmental hazards - many waste rock piles are acid generating, and may contain various contaminants such as arsenic.
Ore is crushed and ground to a fine sand and processed in the mill to extract the mineral. It is at this stage that the company will add chemicals, reagents such as ammonia, chlorine, hydrochloric acid, sulphuric acid, cyanide, and sometimes mercury (depending on the metal that is being extracted) to get the mineral. The wastes from this process are called tailings or "slimes". Tailings are kept in "ponds", which can extend for several kilometres. There are instances of tailings dams failures where the toxic soup that is kept in the tailings ponds is released into rivers and lakes. Humans, wildlife and fish can be seriously harmed, and sometimes killed.
It is estimated that the Canadian mineral industry generates 1 million tonnes of waste rock and 950,000 tonnes of tailings per day, totaling 650 million tonnes of waste per year. This is more than 20 times the amount of municipal solid waste generated each year by all the residences, industries, commercial establishments, and institutions in Canada combined.