RDU 12 – Slag

Established in 1998, the Anaconda Regional Water, Waste & Soils (ARWWS) Operable Unit (OU) is the largest and most diverse Operable Unit in the Anaconda Smelter NPL site. As a result, the OU was separated into fifteen Remedial Design Units (RDUs) based upon factors such as location, source and type of contamination, and remedial needs. Click here for more information about the ARWWS OU.

A by-product of the smelting process, slag consists mainly of copper sulfide, copper-iron sulfide, and copper-arsenic sulfide. Remedial Design Unit (RDU) 12 – Slag consists of three sites the main Granulated Slag (MGS), Anaconda Landfill Slag (ALS), and West Stack Slag (WSS). Over 40 million tons of slag was produced during the 100 years of copper smelting and refining in Anaconda. The black, glass-textured material also contains lesser amounts of zinc, cadmium, lead, and silver. The slag in Anaconda has undergone a process which chemically fixes the metals and contaminants in a glassy, iron silicate, thereby greatly reducing the mobility of the contaminants and the toxicity of the slag particles. All sites within RDU 12 are located approximately one mile east of Anaconda. Physical access to these sites is restricted due to safety and human health concerns.


The only visible slag pile in Anaconda, the Main Granulated Slag (MGS) site, is a 168 acre site containing approximately 41.3 million tons of slag. The MGS is situated on the western edge of the Deer Lodge Valley, just southwest of the MT Highway 1 and Highway 48 intersection. The Anaconda Landfill Slag (ALS) site is a 16 acre site with an estimated volume of 129,000 cubic yards of slag. The ALS is located north of Warm Springs Creek and west of the Drag Strip Area.

The West Stack Slag Site (WSS) consists of two slag piles located in a gulch west of the smelter stack. The estimated amount of slag is about 56,000 cubic yards based on the Record of Decision (ROD); however, according to more recent topographical data, the estimate is closer to 360,000 cubic yards or approximately 540,000 tons. The two piles in the WSS site occupy about 13 acres of land.

Research has discovered numerous avenues by which slag can be repurposed as an economic resource. However, all reuses of slag must be approved by the EPA and comply with the standards of the Occupational Safety and Health Administration (OSHA), the Montana Department of Environmental Quality (MDEQ), and the Resource Conservation and Recovery Act (RCRA). Remedial action for the three sites within RDU 12 includes repurposing the slag as a resource rather than a waste, while preventing human exposure to waste materials and contaminants through proper waste and debris handling/disposal and controlled access.


US Minerals facilities

In the fall of 2012, Atlantic Richfield entered a contract with US Minerals, one of the leading manufacturers of slag products in the United States. Though based in Indiana, US Minerals operates across the nation from six processing locations, including Indiana, Illinois, Louisiana, Texas, and now Anaconda, MT.

The contract between ARCO and US Minerals allows US Minerals to process and sell the Main Granulated Slag Pile for use in the roofing and abrasives markets, both EPA-approved uses for slag. Currently, the existing processing plant in Anaconda is being upgraded. Once upgrades are complete, the slag will be processed in Anaconda. This includes a drying and screening process, after which the slag is separated into three categories: fine, medium, and coarse. The coarse slag is used in the roofing industry to create asphalt roofing shingles, while the medium and fine grade slag is used for sandblasting; all EPA approved uses. The amount of slag that will be transported and processed is yet unknown, and largely depends upon demands within the roofing and abrasives markets.

Though the US Minerals plant in Anaconda is still in its first year of operation, a total of six jobs have already been created. The plant will be in full operation within the year, and, at that time, US Minerals will begin to seek filling a total of ten to twelve positions. Primary focus is presently upon upgrading the processing plant and courting customers.

Aside from the roofing and abrasives markets, there are a plethora of other potential uses for slag. Slag has been used as a substitute for sand in cement, yielding a water-resistant product of increased strength and durability. It is also used in the production of asphalt to create high stability, skid-resistant pavements. These are all EPA approved uses of slag. Another EPA approved use of slag is one with which Anacondans are familiar; in controlled landscaping, such as golf course sand traps.

International interest in the re-use of slag has spurred numerous studies investigating revolutionary new methods of repurposing slag. One of these studies found that slag can be used to create inorganic brown or black pigments for ceramics. Calcination – a thermal treatment process used in the decomposition of solid materials – was employed to extract these pigments from the slag. Though these new possibilities are exciting, such reuses of slag have not yet been approved by the EPA.

Visit the Superfund Library, located on the Arrowhead Foundation’s website, to review documents and research regarding the Anaconda Smelter NPL site. Additional information can also be found on the EPA’s website.