The DeLamar Silver Mine is a Joint Venture between Earth Resources Company, the Canadian Superior Mining Company and the Superior Oil Company. The property is a consolidation of several old operations in the district. The more widely known are the DeLamar and Sommercamp Mines.
The location is 20 miles northeast of Jordan Valley, Oregon and 55 miles southwest of Boise, Idaho. The mine is on the south-slope of DeLamar Mountain in the Owyhee-Mountains of southwest-Idaho.
Initial interest started in 1969 with serious-exploration beginning in 1970. The feasibility study was completed in 1974, and in 1975 Mountain States Engineers were retained to design and build the plant. Processing of ore began in March of 1977 with the first bullion poured in April of that year.
The plant is currently processing some 2200 TPD of ore with a grade of four ounces of silver and 0.03 oz. of gold per ton of ore. Cyanide consumption is 2.5 lb/ton, zinc consumption of 0.3 lb/ton and flocculent consumption of 0.5 lb/ton. A flowsheet is shown as Figure I.
The grinding circuit is of two stage design. The primary mill is an 18 ft. dia. by 9 ft. long semi-autogenous unit. The secondary mill is a 9½ ft. dia. by 15 ft. long overflow ball mill. The mill discharges are combined and sent to 15 in. cyclones for classification. Cyclone overflow reports to leaching with the underflows reporting to the ball mill. The cyclone feed pump is an 8×6 metal case pump.
The primary mill has 1000 installed horsepower and rotates at 68% of critical speed. The ball mill has 700 installed horsepower and rotates at 72% of critical speed.
Because the primary mill is horsepower limited the ball charge is varied to maintain the desired throughput. The mill power is held at 980-990 HP by modifying the feed rate. Four inch forged steel balls are added any time the ore feed fails to reach 100 tons per hour for four hours. During periods of extremely hard ore, the ball charge has been as high as 14 v/o and at other times the mill has operated fully autogenous for weeks.
The main point being that due to extreme variability of DSM ore the ball ration changes daily. If the ore suddenly goes from very hard to very soft, the cyclone underflow is recycled to the primary mill to maintain a liner protecting load until the ball charge is ground down or the mine can supply some competent rock. This also reduces the load on the ball mill and prevents overloading the circuit. The other extreme case is a sudden change from hard to clay rich, soft ore. In this instance, all of the cyclone underflow is put to the ball mill and several tons of balls are charged to the primary mill. Obviously, during normal operations some intermediate combination of the above is used.
The ball mill ball ration is varied to maintain the static charge level two inches above the discharge trunnion line. This results in a ball mill power draw of 550 HP.
Ideally, one would feed an ore mixture such that fully autogenous grinding was possible at the desired feed rate. This luxury is not available at DSM. Between the extreme variability of the ores and the operating constraints of the weather, the mine has not been able to supply a blended ore feed. During the very dry and very cold seasons the mill feed is predominately high clay ores. During the other seasons, the feed stream is predominately rocky to facilitate materials handling problems due to mud. Even with these feed pertubations, it is necessary to shut down the plant for an hour each shift to clear the belt lines and transfer chutes of mud during the fall and spring seasons.
As a result of the high (+30%) clay content of DSM ores; pulps are quite viscous. This unusually high viscosity results in decreased throughput and impaired classification. A viscosity reducer is added to the primary mill and carries through the entire circuit. The most dramatic effect is to allow a higher cyclone overflow density with no change in classification, i.e., 38% solids vs. 28% solids. The increased density results in a significant increase in leach tank residence time. This is beneficial in that increased residence time helps compensate for high throughput surges from the grinding circuit. Some typical grinding circuit data are shown in Table I.
Over the past three years since starting the primary mill liners have gone through a steady evolution. The high wear areas have been identified and the liner sections modified to extend life. Presently, the grates are of pearlitic chrome-moly with a hardness of 321/375 BHN. The balance of the liners are of martensitic chrome-moly with hardness of 363/444 BHN.
The changes in liner design are shown in the following slides.
Typical liner life is estimated as follows:
Feed End Plates……………………400,000 Tons
Feed End Clamp Bars……………400,000 Tons
Cylinder Plates……………………..300,000 Tons
Cylinder Clamp Bars……………..150,000 Tons
Grate Clamp Bars…………………300,000 Tons
Due to the continuing changes in liner sections we do not have exact numbers for liner life and scrap loss. Liner life varies widely with ore characteristics. When milling “hard” ore with a “high” ball charge liner wear is much faster than when milling “soft” ore with a “low” ball charge.
In conclusion the use of semi-autogenous grinding at DeLamar Silver-Mine has been most successful. A conventional two or three-stage crushing plant followed by rod mill-ball mill grinding would be unable to cope with the wet clays during the spring and fall seasons.
The unexpected presence of a high percentage of clay has had one beneficial effect. The average power for grinding has been 12.5 KWH/T rather than the design value of 15.5 which results in the throughput being some 300-500 tons per day over design.