Pit yields large copper output– Asarco finds Mission

The Mission complex, just south of here, represents the combination of a number of mining interests over the years into a single property controlled by Asarco (NYSE).

With the expansion of milling operations completed in 1991, the Mission complex now produces about 124,000 tons of copper in concentrate per year (248 million lb.), ranking it as Asarco’s second largest producer after the company’s Ray complex (T.N.M. Nov. 23/92).

Mission operations, recently toured by The Northern Miner, are currently mining about 200,000 tons of ore and waste per day from a single pit measuring about 2.5 miles long, 1.5 miles wide and about 1,100 ft. deep. Proven and probable reserves currently stand at about 585 million tons grading 0.67% copper with a strip ratio of 2.68-to-1 at a cutoff grade of 0.3% copper.

The company’s recent expansion, representing a production increase of about 47%, involved the rehabilitation of the old Pima No. 2 mill as well as the expansion of the mining operations.

Asarco bought the Pima mill from Cyprus Minerals in 1986 for about US$6.25 million, after the purchase of the Cyprus mining interests in 1985 for US$12.5 million.

The entire rehabilitation and expansion cost US$125 million, including about US$33 million in mill upgrades, 33 million tons of prestripping plus the purchase of 19 new 200-ton haul trucks and three new 28-cu.-yd. electric shovels.

The new mining equipment adds to an existing fleet of four electric shovels (two 15 cu. yd. and two 17 cu. yd.) and 34 170-ton trucks.

Sy Lakosky, mine manager, said the company has looked at the feasibility of installing an in-pit crusher and conveyer system but the return was only about 4%. This, coupled with the logistical problems of blending as well as the different mill locations, makes a standard truck-shovel mining operation more feasible.

The old Pima No. 2 mill, now known as the South mill, has an operating capacity of about 19,000 tons per day. That, in concert with Asarco’s existing Mission mill with a capacity of about 40,000 tons per day, puts total mill capacity at about 59,000 tons per day.

Mission copper output in the first nine months of 1992 was below expectations at 75,880 tons, primarily as a result of lower throughput because of harder ore.

The primary ore mineral is chalcopyrite which is hosted in nine distinctive types of rock varying in hardness from a work index of six, up to a work index of 20.

During the past six years, Mission ore has had an average work index of about 12 compared with current levels at 16-18.

Fortunately, the startup of the South mill has allowed the company to begin blending the various types of ore.

The South mill utilizes two semi-autogenous grinding (SAG) mills in concert with two recycle cone crushers. The large diameter SAG mills at the South mill work much better than the old Mission mill’s gyratory crushers and rod-mill/ball-mill combination in treating the harder ores.

As a result, Asarco started blending ores in July, sending a larger proportion of harder ore to the South mill.

In a further effort to deal with the harder ore, the company is now reviewing plans to increase secondary crushing capacity at the Mission mill by boosting horsepower.

Asarco is also reviewing the installation of a Distributive Control System (DCS) in the Mission mill to boost output and lower costs. The South mill already operates with DCS, a system which essentially provides real-time monitoring of the entire milling operation.

The company estimates that the system boosted South-mill production by 10-12%. The South mill uses rougher flotation and a regrind followed by column cell flotation, while the Mission mill utilizes cleaner cells following the regrind.

Recoveries at the Mission and South facilities are currently running at about 88% and 87% respectively.

For the first nine months of this year head grades have averaged about 0.62% copper putting cash operating costs at about US77 cents per lb. and total costs at about US91 cents per lb.

Asarco expects head grades to average about 0.67% next year, helping to lower projected cash costs to about US66 cents and total costs to about US80 cents. Concentrate is trucked a short distance to a rail load-out facility and then railed to either Asarco’s El Paso smelter in Texas or the Hayden smelter south of Phoenix.

The Mission complex is not currently operating any heap-leach or dump-leach solvent extraction plants. The company is, however, reviewing future plans for a solvent extraction and electrowinning operation. Mission has about 10 million tons of oxide ore stockpiled at various points throughout the operations.

In addition, Asarco has outlined about 20 million tons of oxide reserves, which will be mined during future pushbacks in the current pit. The grade of the oxide material averages about 0.50% copper.

Asarco is also reviewing the Mission ore reserve data with an intention of increasing reserves.