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Gold Mine

 

Overview

When you think of the gold rush days in the Old West, you probably picture the miner as a grizzled prospector with his mule and pickax. Gold mining today, however, is a scientific process that uses computers, geologic data, chemistry, microbiology, and sophisticated refining equipment to extract trace amounts of gold from rock blasted out from deep underground. If you look at a map of an underground mine, such as the one that the Homestake Mining Company operates in South Dakota, it looks like a very orderly ant farm, with rooms carved out of the solid rock for machine shops, laboratories, and other facilities. Data from geological core samples goes into a computer that makes a drawing of an area (like a connect-the-dot map in three dimensions) and tells the miners where to find gold-bearing rock. The miners then drill a series of precision holes into the rock face, pack in explosives, clear everyone out, and blast. After checking for gas leaks, workers reinforce the walls and ceiling to prevent cave-ins and then hoist the ore out of the mine through the vertical shafts. Gold, a pure element identified with the chemical symbol Au, exists in nature. (Fool's gold, a compound of iron and sulfur called a pyrite, looks a little like gold.) Tiny gold particles are encased in tons of rock, so the ore first goes to a mill where it is crushed very fine. The larger particles separate from the ground rock on a vibrating table that works on the same principle as panning, in which substances of different densities separate from each other. The smallest particles of gold then are dissolved (leached) out of the ground rock with a weak cyanide solution. This still doesn't get all the gold, and it leaves some very toxic wastewater behind, but miners have some valuable new helpers for both of these problems: bacteria. Some bacteria chew up the cyanide in wastewater. Others chemically alter stubborn rocks so that the cyanide treatment can be more effective. This is called bioleaching. A side benefit of bioleaching is the fast production (and consequent treatment) of acids that would otherwise leak slowly from the sludge into the environment.

Activity

Cave-ins have always been a serious danger in mines. As the depth increases, the pressure on the walls and ceiling of the mine tunnel becomes enormous. Engineers have worked to design new methods of reinforcing tunnels so they will not collapse. Using just paper as a reinforcement material, can you design a good, safe tunnel? Try this as a contest among several groups of students. Materials
  • two cardboard boxes, one small enough to fit inside the other with considerable room to spare, but deep enough to hold several inches of sand. The small box should have a hole cut at the base in two opposite sides, large enough for the experimental tunnels to slide through (see diagram).
  • several pounds of wet sand
  • a big spoon, scoop, or measuring cup
  • several sheets of regular white paper or construction paper
  • tape 1. Place the smaller box, with the open top facing up, inside the larger box. (The larger box is just there to catch spilled sand.) 2. Design a tunnel, using only paper with enough tape to hold it together. Start with something simple like a long, narrow box or tube. 3. Place the tunnel through the holes in the small box. (The tunnel must be long enough to fit through both holes with an inch or so protruding from each side.) 4. Pour wet sand into the small box in measured quantities (scoops or cups), covering the tunnel. Record how much sand is required to make the tunnel collapse. (You will have to look through the tunnel from one end to determine when it collapses.) 5. Design some reinforcements or cross braces for your tunnel, still using just paper. You might try folding, twisting, tightly rolling, or braiding pieces of paper to obtain supports with different strength characteristics. 6. Repeat steps 3 and 4 to test your design again. Questions 1. Which design best resisted collapse? Was any particular cross section-triangular, circular, rectangular-unusually good at withstanding pressure from all sides? Was any design better at withstanding pressure from the side than from the top or vice versa? What kind of cross braces were most effective? 2. Miners usually don't build tunnels and then bury them-they dig tunnels underground. Which tunnel reinforcement design would be easiest to install if you packed the small box firmly with sand and then dug a tunnel? Can you think of a way to dig and reinforce at the same time?

    • Resources

      Science, p. 778.
      Parker, W. (1966) Gold in the Black Hills. Lincoln, NE: University of Nebraska Press.
      Sawa, T. (1996, Dec 30) Yukon alchemy: A mine raises environmental concerns. Maclean's, p. 92.
      Computer software:
      MECC: The Yukon Trail.CD-ROM.
      (800) 685-6322 or (612) 569-1500
      Nissen Ventures: StripMiner for Windows.
      (604) 436-5501

      Web Sites
      Homestake Mining Company
      199.170.0.164
      Info-Mine
      www.info-mine.com/
      Videos (free) on acid mine drainage are available on request from:
      www.info-mine.com/technomine/
      enviromine/videos/vmain.html
      O&K Gold Mining Game
      www.oandk.com/game1.html
      Requires Netscape 3.0 plug-in for Shockwave.
      Recreational Gold Prospecting
      and Mining Page
      www.dnai.com/~wfw
      One link goes to a list of organizations
      (categorized by state) devoted to
      recreational gold prospecting.