Maybell Mill Site
Moffat County, Colorado

Location:   The Maybell uranium mill site is located 5 miles northeast of the town of Maybell in Moffat County, Colorado.

Background:   In November 1953, privately funded airborne radiometric surveying located anomalies over outcrops of fluvial sandstone strata of Miocene age near Maybell, Colorado. Only a few weeks earlier, radioactive outcrops had been located in similar strata near Baggs, Wyoming, during U.S. Geological Survey geological reconnaissance/radiometric surveying conducted for the U.S. Atomic Energy Commission (AEC). By March 1954, follow-up private airborne surveys, surface prospecting, and subsequent drilling had defined economic sandstone-type uranium deposits near Maybell. In August 1955, the AEC entered into a contract with the Trace Elements Corporation (TEC) for a uranium mill at Maybell, Colorado. TEC established a pilot-scale, ore-processing plant for test extractions and recovered 238 pounds of uranium concentrate, which the AEC purchased under the initial contract. In November 1956, the initial contract was superceded by a new AEC contract for a full-scale uranium mill near Maybell. Over the next year, TEC built the mill on an 84-acre tract near the uranium deposits. In late 1957, before the mill was placed in operation, TEC became a subsidiary of Union Carbide Corporation (UCC, later UMETCO), which assumed control of the operations. The first delivery of concentrate from the mill was received by the AEC in December 1957. In February 1962, TEC was fully merged into the Union Carbide Corporation, which assumed the TEC rights and obligations under the AEC contract. All uranium concentrate produced at the mill was sold to the AEC. Ore for the mill came from near-by open pit mines. In size, the ore deposits ranged up to 250,000 tons maximum. Output of ore from the open-pit mines varied from about 750 to 1,000 TPD of all grades. The average grade was about 0.20 percent U₃O₈, though the grade was highly variable, and economic ore was mined concurrently with large tonnages of low grade ore. The mine output was classified into four storage categories that ranged from mill-grade ore (at least 0.13 percent U₃O₈) down to waste rock (less than 0.02 percent U₃O₈). Over the years 1957-1964 under three AEC contracts, the mill processed nearly 1.8 million tons of ore that averaged 0.13 percent U₃O₈. The mill was operated at a throughput rate of about 700 tons per day (TPD), and its average uranium recovery rate was 88 percent. The mill was unique in that it used an upgrader circuit to treat low-grade ores before leaching. Low grade ore (containing less than 0.20 percent U₃O₈) was treated in the upgrader circuit that separated the uranium-bearing slimes from the sand fraction, which was rejected to the tailings pile. The slimes fraction was then acid leached. Higher grade ore was treated directly by acid leaching. The leached slurry passed through a classification/washing circuit that used parallel spiral classifiers and cyclones to separate the fine-grained slurry containing the uranium. This slime product was combined with the leached slimes from the upgrader circuit before entering ion exchange (IX) circuit. The resin-in-pulp ion exchange circuit consisted of six adsorption stages and ten elution stages. The continuous, countercurrent resin-in-pulp process was the first application of this technology in the domestic industry. Uranium was washed from loaded IX resin using ammonium nitrate solution and was precipitated directly with anhydrous ammonia. The coarse fraction from both circuits went to tailings storage. When the mill facility was closed down in October 1964, about 2.6 million tons of mill tailings remained at the site in a pond covering some 80 acres. In the late 1970s, UCC began heap leaching of the remaining low grade ore pile stockpiled at the site. A portable ion exchange plant was used at the site to collect the uranium from the heap-leach solution. The plant eluate product was trucked to the firm’s Gas Hills uranium mill for final processing. By 1982, the mill was dismantled, and the tailings pile was contoured and stabilized with a 6-inch covering of soil that was seeded with native grasses.