Greens Creek | processing
At Greens Creek, the ore is processed on site at a mill, which produces lead, zinc, and bulk concentrates, as well as gold doré. In 2011, ore was processed at an average rate of approximately 2,115 tons per day. During 2011, mill recovery(1) totaled approximately 73% silver, 79% zinc, 68% lead, and 62% gold. The doré is further refined by precious metal refiners and sold to banks, and the three concentrate products are sold to a number of major smelters worldwide. Concentrates are shipped from a marine terminal located on Admiralty Island about nine miles from the mine site.
(1) Excludes zinc to the lead concentrate and lead to the zinc concentrate.
There are three main phases to the milling process: grinding, flotation, and filtration.
grinding – the reduction phase
Grinding is the reduction of the mined ore from an average size of 9” (225mm) to an average size less than 50µm (50microns or .05mm). The main purpose of the grinding process is to liberate the various components of the ore. A 9” lump of ore contains some lead, zinc, and silver minerals, some elemental silver and gold, but mostly a variety of other gangue minerals that are not economic to recover, what we typically consider ordinary rock. By grinding that lump down to particles of 50µm, we aim to produce mineral particles containing zinc, lead, silver, or gold that can be separated from the gangue and each other. Water is added during the grinding phase producing slurry that can be pumped and minerals separated in the flotation process that follows.
flotation – the separation phase
Flotation is used to separate the mineral particles liberated during the grinding phase. Reagents are added to encourage or depress the tendency of specific minerals to float in water. The ground ore and water slurry is passed though a series of agitated cells where air is injected to produce bubbles that selectively float minerals to a silver rich lead concentrate, a zinc concentrate, and a bulk concentrate that contain a mixture of more difficult to separate zinc and lead mineral particles. The silver and gold are more difficult to isolate and some ends up in each of the concentrates as well as in a small amount of gold / silver concentrate which is smelted on site to produce a doré bar.
filtration – the refinement phase
Filtration is used to remove most of the water from the concentrates and tailings. All of the mill products are filtered to produce a dry cake with a moisture content of around 11%. The filtered concentrate is trucked to the port storage area and then shipped to smelters around the world to be turned into metals and other products. About half of the filtered tailings is combined with cement and used underground to backfill and support the mined-out areas. The rest of the tailings are trucked to an engineered and regulated dry-stacked tailing disposal facility near Hawk Inlet.
