Ammonium Nitrate Fuel Oil Plant
When the Anaconda Company began developing the Berkeley Pit in 1955, it quickly realized that a new type of explosive was required to break up the large volume of rock that had to be moved‒greater than 200 million tons between 1955 and 1963. The blasting agent ammonium nitrate-fuel oil (ANFO) quickly supplanted dynamite as the explosive of choice, not only in the Pit (beginning in about 1958) but also at Butte’s underground mines (beginning about 1961).
During the late 1950s, industry wide the use of ANFO reduced the cost of explosives used in mining 50-60%. By 1975, 90% of all pit blasting in the United States was accomplished with ANFO-based blasting agents.
Anaconda Company was a regular innovator in customizing mine-related equipment and techniques at the time. Its early use of ANFO and the equipment it developed for its mixing and placement can be attributed in part to the scale of the company’s combined underground and open-pit mining operations.
The company chose a site for the delivery and mixing of ammonium nitrate and diesel oil at some distance from both the Berkeley Pit and its then-active underground mines—the former Speculator mineyard. There, the Butte, Anaconda & Pacific Railway delivered bulk fertilizer-grade ammonium nitrate prills (pellets), trucks delivered No. 2 diesel, and the work of making ANFO commenced. The bulk ammonium nitrate was dumped into an underground chute at the edge of the railroad track, then conveyed by elevator to a pair of tall storage bins. Two large fuel tanks on high ground at the south end of the plant contained diesel that was allowed to flow through two 2-inch pipes encased in a wood box that led to the lower level of the plant.
If destined for use in the Pit, the bulk ammonium nitrate was loaded by gravity from the bottom of the storage bins into the bed of a truck that was outfitted with a large, covered bin and a separate 10,000-pound diesel tank. The truck immediately headed to the Pit where the two ingredients were mixed just prior to filling the drill holes.
If the ANFO was to be used underground, the materials were mixed together using a converted cement mixer barrel in the mixing shed—a large metal building at the ANFO plant. Workers then bagged the ANFO in 25- and 50-pound polyethylene or coated Kraft paper bags and immediately loaded them on a truck to be delivered to a mine, where it was briefly stored underground in storage magazines. The practice of using ANFO as a blasting agent underground ended at an unknown date prior to 1974 when Butte’s last underground mine closed.
The ANFO plant continued operations at the Berkeley Pit until the Anaconda Company, then owned by the Atlantic Richfield Company, permanently closed its mining operations on the Butte Hill in 1982. When the ANFO plant was abandoned, many of its working structures were left in place. Today, those structures that remain include the far end of the BA&P railroad track that led to the plant on the upper level; the chute, gates, and gate control shed that regulated the flow of ammonium nitrate to the storage bins on the middle level; and on the lower level the base of storage bins structure. Between the upper and middle level, a wood staircase for worker access continues to stand, in a deteriorated condition. Salvagers removed the mixing shed and its equipment decades ago.
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