Milling and concentrating nonferrous metals from ore material mined through open pit and underground mining methods removes most of the metals and generates sterile crushed rock of uniform size having little or no organic value. This crushed rock, commonly known as "mine tailings" is normally deposited in surface impoundments or tailing dams. Tailings generally contain metals in a bound, non-extractable form, and a significant degree of moisture (up to 15%). Tailing dams or impoundments can range in size from several acres to thousands of acres, and can be up to hundreds of feet high.
Generally, the tailings are deposited on or near the mine site in plain view which tends to create aesthetic problems. Moreover, the tailing dams create environmental concerns generally related to dust and water erosion problems. In particular, because the mine tailings constitute very fine rock particles, wind can create the possibility of dust storms. In addition, severe rainstorms or other weather conditions can erode the tailing dams, washing the fine rock particles down into streams or other water sources. Because the tailings are high in metal content, including lead, having them washed downstream or breathed by people may be hazardous.
Most state reclamation laws in the United States require the preservation and reapplication of original top soil or "best available growth media" on mine waste or mill tailings to facilitate revegetation. Conventional reclamation practices include soil covering, hydromulching, and/or applying crushed rock to the tailing piles.
Soil covering generally consists of applying a layer of soil over the top of the tailing dams to prevent erosion of the tailings. The soil can be either preserved from the mine when it is initially dug, or borrowed from the top soil of other areas. The soil covering technique is intended to support vegetation to control erosion of the soil, influence water infiltration into the tailing dams. provide a medium for root penetration, and provide mechanical strength for the long term.
Hydromulching, which is often used in conjunction with soil covering, is the process of combining mulch, fertilizer, seed, a binding agent such as tachifier and water into a sprayable mixture, and spraying that mixture onto the soil cover to induce the growth of vegetation and to keep the soil cover in place.
Several problems with soil covering and hydromulching exist. First, it is very expensive to haul large amounts of top soil onto the tailing dams and then apply the hydromulching process. The mining companies must remove soil from other areas, load it onto trucks, spread it on the dams with ground moving equipment, and then apply the hydromulching process. Second, the top soil generally must be mined or borrowed from other places. Third, because the tailings and the soil are of different consistencies, they do not blend, and after several years, the soil tends to erode and fall off the dam slopes in large pieces. Finally, soil covering techniques can only be effectively used on inactive tailing dams. Tailings are placed in the impoundments or dams while they are still very wet from the mining process. As new tailings are added, additional water is added as well. If a soil cover is placed on an active dam while new tailings (and water) are added, a perched water table tends to evolve, causing a very instable structure. In some cases, water pressure building beneath a soil cover will actually blow the soil cover completely off the dam. Obviously, many problems can arise in such a case.
The other conventional erosion control technique consists of applying a thin layer of crushed rock over the tailings to prevent them from blowing and/or washing off. After the rocks are applied, the crevices and gaps between the rocks can trap wind blown seeds allowing for some vegetational growth on the dams. However, as with soil covering, the crushed rock approach has several negative aspects. First, aesthetically, the site appearance is not generally enhanced as the rock tends not to be much better than the tailings. Further, unless a substantial amount of rock is applied, and generally reapplied over time, the application tends to be nothing more than a temporary solution. Moreover, because both the rock and the tailings have little or no organic value, even if seeds are trapped between the rocks, very few if any will germinate and grow. Finally, as with soil covering, hauling large amounts of crushed rock onto the tailing dams can be very expensive.
Tailing dams also tends to contain and hold water, something like a sponge, for very long periods of time. However, because tailings provide little or no organic value, plant life generally cannot be sustained.
As will be appreciated, there thus exists a long felt need for an inexpensive, yet effective method of controlling erosion and building a growth medium that will sustain vegetation on tailing impoundments or dams, as well as on decertified areas of the desert, areas damaged by poor grazing practices, and other generally disturbed areas of the world.