CHM 110 - CHEMISTRY AND ISSUES IN THE ENVIRONMENT
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Asarco Mining Operations in Arizona
 
Mission Mine, Tucson
Text edited by Rob Vugteveen, Director, Asarco Mineral Discovery Center

 Mission Mine

The Asarco Mission Mine is about 15 miles south of Tucson, Arizona. It is 2 miles long, 1.75 miles wide and a quarter mile deep. The copper ore "contains" chalcopyrite (and not that much of it) is 0.67% copper, which means that 13 pounds of copper are produced from every ton of ore. In addition, about 3 tons of waste rock need to be removed to recover one ton of copper ore. Each year this mine produces 475,000 tons of concentrated copper ore (28% Cu), which eventually comes to 130,000 tons of pure copper metal, and 2 million ounces of silver.

Prof O: chalcopyrite
Prof O: Mission Mine
Prof O: Mission Mine - waste dump and "benches"
Asarco mining link
 
Almost on a daily basis one of the mine operations is to drill holes for blasting to loosen the rock for the shovels. The blasting is done with a mixture of ammonium nitrate and fuel oil (ANFO). As an aside, one blasting hole uses as much ANFO as was used in the Oklahoma City bombing.
The large shovels are powered by electricity, pictured in the above photos. Three scoops of ore or rock fill the very large trucks, which may hold 240 tons, four scoops for a 320-ton truck. The tires are 11 feet in diameter. As a side note, many of the trucks are driven by women.
 

Ore Crushing Operations

Outside of the Mission South Mill, trucks dump the ore into the primary crusher which reduces the rocks to 8 inches or smaller. Inside the mill, the ore is mixed with water in two rotating SAG mills which use the larger rocks and 8-inch steel balls to reduce the ore to about 10 mm or smaller. Then two ball mills grind the ore to about 0.2 mm with 3-inch steel balls. This copper ore slurry is finally pumped to the flotation deck.
 
Prof O: Ore Crushing Operation
Prof O: Ore Crushing -detailed
Asarco milling link -rotary mills
 

 Ore Concentration by Froth Flotation

Remember that only 0.67% of the ore is copper. The copper minerals and waste rock are separated at the mill using froth flotation. The copper ore slurry from the grinding mills is mixed with milk of lime (simply water and ground-up limestone) to give a basic pH, pine oil (yes, it comes from trees -- a by-product of paper mills) to make bubbles, an alcohol to strengthen the bubbles, and a collector chemical called potassium amyl xanthate (or the potssium salt of an alkyl dithiocarbonate). These are added to the slurry in relatively small quantities. Xanthate is a long hydrogcarbon (5 carbons) chain molecule. One end of the chain (the ionc dithiocarbonate) is polar and sticks to sulfide minerals while the other end is nonpolar, containing the hydrocarbon chain is hydrophobic -- it hates being in the water and is attracted to the nonpolar hydrocarbon pine oil molecules. Raising the pH causes the polar end to ionize more and to preferentially stick to chalcopyrite (CuFeS2) and leave the pyrite (FeS2) alone. Air is blown into the tanks and agitated like a giant blender, producing a foamy froth. The chalcopyrite grains become coated with xanthate molecules with their hydrophobic ends waving around trying desperately to get out of the water. They attach themselves to the oily air bubbles which become coated with chalcopyrite grains as they rise to the surface and flow over the edge of the tank. In this manner through a series of steps the copper ore is concentrated to an eventual value of over 28% copper. Waste rock particles do not adhere to the bubbles and drop to the bottom of the tank. The waste material that comes out of the bottom of the tanks at the tail end of this process is called "tailings." It is nothing more than ground-up rock with the copper minerals removed.
Prof O: Ore Concentration by Flotation
 
 

Water Reclaimation and Tailings

The concentrated ore, now called copper concentrate, is dewatered and dried to about 10% moisture content. It is shipped by truck to the Asarco smelter in Hayden, Arizona. (The El Paso smelter is on stand-by status). Water is recovered from the tailings in another thickener. The thickened tailings flow by gravity in large pipes to tailings ponds, where the solid material settles out and additional water is recovered and pumped back to the mill. About 80% of the water used at the mine is recycled and re-used. The rest is lost to evaporation and used to keep haul roads damp to minimize dust.

After a tailings dam is decommissioned, the tailings are capped with dirt and treated with manure for grazing cows or ammended with biosolids (sludge) from a local waste water treatment plant. This provides organic material and nutrients to create a sustainable soil for native grasses and shrubs.

Prof O: Final Settling Pond

 
 
 
To continue the story of copper refining a similar operation of Asarco is at the Hayden, AZ smelter.
 
Ray Copper Mine and Hayden Smelter
 
Hayden Copper Smelter Home Page - graphic
 
Ray Copper Mine is located about 20 miles south of Florence, Arizona. Photos by C. Ophardt.
 
ProfO: Ray Copper Mine graphic - 96K
The above graphic shows the extensive amount of overburden, rock, and tailings from the mining operation. The pictures also show how the tailing banks erode, even though the climate is quite dry.
 
ProfO: Ray Copper Mine graphic - 152K
 
The above graphic (taken with a telephoto lens) shows the extensive "hole" that is dug - hundreds of feet deep and perhaps up to 3/4 mile long. A close up insert also shows the relative size of the large "shovel" and truck to haul the crushed rock. The truck tires are 10-15 feet in diameter to give an idea of the scale.
 
ProfO: Hayden Smelter graphic - 96K
 
The above graphic shows several buildings associated with the Hayden Copper Smelter. The blue "pools of water" are actually sulfuric acid solutions containing copper sulfate concentrate. Copper sulfate is a bright blue color. The smelter is the building with the tall smoke stack which was originally built to simply disperse the sulfur dioxide gas higher into the atmosphere where it would be dilute more down wind. This was the first method of air pollution control, which was really no solution at all except for areas near the plant. More extensive pollution controls are now used to "scrub" or remove the sulfur dioxide from escaping. This sulfur dioxide in now converted into sulfuric acid which is a valuable by product. At the Hayden smelter, 2,500 tons of sulfuric acid are produced each day.

Prof O: Copper Smelting Process
 
Asarco smelting link
 
The 99% pure copper from the anode furnace is cast into 750-pound, 3-inch
thick, rectangular slabs called anodes.
 
Scrubber Example from the El Paso Plant - 98% sulfur dioxide in now converted into sulfuric acid
Capture of sulfur dioxide, conversion to sulfuric acid, and use of sulfuric acid - BHP Innovation Site

Series of graphics that show entire copper mine to copper metal production - BHP Innovation Site
 
 
Copper production (short text - no graphics) - Copper Organization
 
Copper Electrorefining - Last step
 
The end result of of the copper smelter operations is anode copper which is 99% pure. This copper is further purified at the Asarco electrolytic plant in Amarillo, Texas. The 99% pure copper from the anode furnace is cast into 750-pound, 3-inch thick, rectangular slabs called anodes. The final step is a process called electrorefining. This is done by electrolysis where the copper anodes from the smelter become positive electrodes and thin, three-foot square sheets of pure copper are used as negative electrodes. Forthy-five anodes and 46 cathodes are interleaved and immersed in an electrolysis tank filled with a solution of sulfuric acid and copper(II) sulfate. Oxidation occurs at the anode, where copper metal is converted to copper(II) ions with the release of two electrons. At the cathode the opposite reaction occurs: copper(II) ions are joined with two electrons to form copper metal. During the migration of metal ions from anode to cathode through the aqueous solution, the impurities drop to the bottom of the electrolysis cell. Some of these impurities are gold, silver, nickel, selenium, tellurium, and platinum which are themselves recovered to be used.
 
After two weeks, the cathodes are pulled from the tank- -- each one about 350 pounds of 99.99% pure copper. These are washed, bundled, and sold. A new set of cathode starter sheets is inserted and the process continues. After two more weeks, the second set of cathodes is pulled and the remainder of the anodes are melted and cast into new full-size anodes.
 
Anode - oxidation: Cu metal (impure)---> Cu+2 + 2 electrons
Cathode - reduction: Cu+2 + 2 electrons ---> Cu metal (pure)
 
 
 
 
 
 
Other methods of concentrating copper from low grade ores Arizona

The leaching of the "oxide" materials is relatively easy and relies on dump, vat and
agitation leaching using dilute sulfuric acid solutions.

Both the Asarco Ray Mine and the Silverbell Mine are leaching low-grade copper with weakly acidic solutions. Silverbell produces 50 tons of SX/EW copper per day (design capacity).
Leaching Copper Ore - Arizona Mining Association
 
Leaching sulfide materials, whether in ore or concentrate, requires a chemical oxidizing
agent ­ ferric ions (Fe3+). These special ferric ions can be generated by reactions with air.
The oxidation can be assisted by either pressure (as in an autoclave) or more commonly
with bacteria. The importance of the bacteria and our understanding of their potential only
introduces an extra variable into the process. But different copper minerals require different
conditions to extract the copper.
 
However, sulfuric acid is not the only reagent that can dissolve copper from concentrate.
For example, BHP has already patented a process using ammonia to dissolve part of the
copper concentrate. (This is the basis of the Coloso cathode production plant in Chile.)
 
The Mission Mine has an an ore body with lots of limestone, which precludes using acid leaching. Leaching with
NaOH was tried here in the 60's with costly success. It worked, but it cost more than the copper recovered was worth.
 
Another type of mining being tried by BHP at Florence, AZ is in-situ copper mining. A weak acid solution is injected in wells to dissolve the copper in the ore without removing it. The concentrated copper solution is then pumped out of recovery wells.
 
Hydrometallurgical routes have become more popular to research because: energy costs are
lower; more pollution-conscious communities require "zero discharge" type conditions;
whether for air or water, as smelting processes are made "leak proof" the impurities become
a problem by concentrating in the copper; hydrometallurgical processes can separate
impurities better; the operating temperatures are lower making pilot plants easier to
operate; sulfur dioxide is a gas that has to be made into acid. (Elemental sulfur is formed
in hydrometallurgical processes.)