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Slick Rock Mill Site and Concentrator Site
San Miguel County, Colorado
| Years of Operation |
Status of Mill
or Plant Site |
Uranium Ore
Processed
(Million Short Tons) |
Production
(Million Pounds U3O8) |
| 1931-1943, 1957-1961 |
Decommissioned |
0.63 |
2.68 |
Remediated
Mill/Plant Area
(Acres) |
Disposal Cell
Area
(Acres) |
Disposal Cell
Radioactive
Waste
Volume
(Million Cubic Yards) |
Disposal Cell
Total Radioactivity
(Ci, 226Ra) |
Disposal Cell
Average Tailings
Radioactivity
(pCi/g, 226Ra) |
UMTRA Project
Final Cost
(Million Dollars) |
| 110 |
12 |
0.86 |
149 |
113 |
50.43 |
Location: The Slick Rock uranium processing sites are about three miles northwest of the old post office at Slick Rock in Montrose County, Colorado.
Background: Two uranium ore processing facilities were operated at different times in the Dolores River Valley near the town of Slick Rock, Colorado. The Slick Rock mill was operated in the 1930s-1940s by North Continent Mines, Inc. Later, in the 1950s-1960s, a uranium concentrator plant was operated by Union Carbide Corporation on a tract about one mile northwest of the Slick Rock mill site.
Slick Rock Mill: In 1931, the Shattuck Chemical Company (SCC) built a mill two miles north of Slick Rock for the production of vanadium and radium salts. Throughput capacity of the original mill was about 15 tons of ore per day (TPD). North Shore Coke & Chemical Company (NSC&C) held a 60 percent interest in SCC from 1934-1942. In the mid 1930s, NSC&C formed the subsidiary, North Continent Mines, Inc. (NCM), to mine uranium-vanadium ores. In 1939, NCM acquired the Slick Rock mill, increased its capacity to 30 TPD, and then operated the mill until it was closed in 1943. In the original Slick Rock milling process, ground ore was acid leached to solubilize the uranium and vanadium values. The solution was classified to separate the uranium-vanadium solution and insoluble radium-bearing slimes from the sand residue solids, which were sent to tailings. The slimes fraction was washed, filtered, dried and trucked for final processing to the SCC plant in Denver, Colorado. Evaporating the acid-leach liquor yielded a uranium-vanadium residue that was then roasted to form iron vanadate: this was filtered, dried, and later converted to ferro-vanadium. Soluble uranium salts in the roasted calcines were dissolved in water, and a low grade uranium concentrate was precipitated, from which the final uranium concentrate product was formed. In 1941, the mill adopted a new process. The ore was treated by salt-roasting, water leaching, and acid leaching to solubilize vanadium and uranium. This process yielded a fused vanadium “red cake” product. The uranium was precipitated from solution and dried and shipped. In the early 1940s, the mill’s vanadium and uranium products were sold to the U.S. Government for the war effort. Information about these sales is not available. The mill was closed in 1943. From 1931 to 1943, an estimated 37,000 tons of ore averaging 0.28 percent U3O8 and about 3.0 percent V2O5 was processed at the mill, and the tailings pile covered an area of about 6 acres on the flood plain of the Dolores River.
In February 1945, Union Mines Development Corporation, which acquired uranium and vanadium for the Government's wartime Manhattan Project, purchased the NCM Slick Rock facility. Union Mines deeded the facility to the Government in 1949, along with numerous uranium mining properties it had acquired in San Miguel County, Colorado. In November 1957, the 160-acre Slick Rock facility, consisting of the mill site, tailings area, and the North Continent mill town site were deeded to Union Carbide Corporation. The Government retained the mineral rights associated with the tract. A small quantity of tailings, about 300 to 400 tons total, was later trucked to UCC's Slick Rock uranium concentrator plant and to its Uravan, Colorado, mill for reprocessing. In 1971, the tailings pile was reclaimed with a 6-inch thick soil covering and seeded in accordance with the then current State of Colorado regulations. By 1982, the mill buildings and structures were dismantled, and equipment and associated buildings were removed from the site.
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Slick Rock Concentrator: Union Carbide Corporation, (UCC) built the UCC Slick Rock concentrator in the late 1950s on a 360-acre tract adjacent to the Dolores River about 1 mile downstream from the old NCM Slick Rock mill site. The concentrator was operated from September 1957 until yearend 1961, and its products were trucked to UCC’s Rifle, Colorado, uranium mill for final processing. The three final products were: “dry-fines,” “green-sludge,” and a “slimes” concentrate. The ore, a coarse-grained sandstone cemented with clay and calcite, averaged 0.24 percent U3O8 and 1.35 percent V2O5. Dry grinding of the ore and air sizing produced a “fines” product (minus-14 mesh) that contained up to 60 percent of the uranium values in the ore. The remaining coarse ore fraction was then acid leached to yield a solution from which the slimes product was separated, dewatered, and dried. The green sludge product was formed by neutralizing a part of the remaining pregnant solution with ammonia. The remainder of the pregnant solution was recirculated. The slimes and green sludge products combined contained about 30-35 percent of the uranium from the ore feed. The concentrator “upgraded” the ore’s uranium values by a ratio of about 4:1, and the combined grades of the “fines,” “sludge,” and “slimes” products was about 1 percent U3O8. The plant operated at a rate of about 430 TPD and treated 591,244 tons of ore containing some 2.7 million pounds U3O8. When the plant closed, the tailings pile contained about 350,000 tons of material and covered 19 acres at the site. The pile was stabilized with a 6-inch thick soil covering and seeded. An earthen dike was built between the tailings pile and the Dolores River, which flowed about 35 feet from the pile at one point. By 1982, the concentrator plant buildings and equipment had been removed from the site.
UMTRA Surface Remediation: Ore processing operations at the two Slick Rock sites resulted in the contamination of the natural surface by wind blown and waterborne materials from the respective plants and tailings materials. In 1995, the residual radioactive materials from each site were trucked to the new Burro Canyon Disposal Cell site. Some 129,000 cubic yards of contaminated materials were relocated from the NCM Slick Rock mill site and about 671,000 cubic yards were relocated from the UCC Slick Rock concentrator site. At each site the excavated areas were back filled with clean soil materials, graded for proper drainage and seeded with native grasses. A total of 17 vicinity properties were cleaned up during the project. The statutory wetlands and riparian areas were reestablished at the two Slick Rock sites. The remediation was completed in 1996.
Responsibility for Remediation: U.S. Department of Energy, 90 percent; State of Colorado, 10 percent.
Stewardship: The NCM Slick Rock mill site, the UCC Slick Rock concentrator site, and the Burro Canyon Disposal Cell site are being managed under the DOE’s Long-Term Surveillance and Monitoring (LTSM) Program in accordance with the approved site specific plans.
Disposal Area: The Burro Canyon Disposal Cell was built by the U.S. Department of Energy (DOE) on a 25-acre tract located about 5 miles northeast of Slick Rock. Formerly administered by the Bureau of Land Management, the tract is now owned by the DOE. The cell occupies about 12 acres of the 61-acre Burro Canyon tract. About 857,000 cubic yards of radioactively contaminated tailings, debris, and soil materials were stabilized in the cell in 1997. Remedial action at the site was completed in 1998. The cell is constructed partly below grade and is about 95 feet deep from its lowest to its highest point. It rises 35 feet above the surrounding terrain. The cell’s cap is a 5-foot thick multiple-component covering. A 1.5-foot thick covering of sandy clay was placed directly on top of the radioactive materials to serve as the radon/infiltration barrier. A 2-foot thick layer of compacted soil was placed above the radon barrier to lessen damage from freeze/thaw cycles. A coarse-grained bedding layer consisting of 6-inches of gravel was added above the freeze-thaw layer to provide drainage and serve as a break in the capillary flow of water. The top slopes and side slopes were covered with riprap to protect against wind and water erosion.
Groundwater Program: Some 95,000 cubic yards of groundwater at the NCM Slick Rock mill site are contaminated with residues from the former uranium-vanadium ore processing activities. A contaminated groundwater plume covers about 19 acres at the site and extends into the shallow aquifers and bedrock beneath the site and offsite. At the UCC Slick Rock concentrator site, about 129,000 cubic yards of groundwater are similarly contaminated: the plume covers 92 acres and affects the shallow alluvial and bedrock aquifers beneath that site. The contaminated groundwater at the two sites has not degraded the water quality in the Dolores River, which flows adjacently to the former mill site. Water from the contaminated portion of the aquifers is not used, and local populations are not threatened by the contaminated groundwater. Natural flushing action of the active groundwater flow will eventually restore the groundwater to applicable water quality standards. It is anticipated that active remediation of the contaminated groundwater will not be required. Long-term stewardship activities are expected to begin in 2008 to assure that natural flushing of the groundwater is proceeding as anticipated. DOE will conduct groundwater monitoring once every three years. When compliance with the groundwater standards is achieved, the site will be released to the owner for unrestricted use.
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