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<br />23
<br />Each plot was planted with a tomato and crested whealgrass, alfalfa, and
<br />tye grain seeds, The plants were allowed to grow for 13 months, by which time
<br />>aost of the wheatgrass, alfalfa, and rye had matured and died and then the
<br />plots containing living tomato plants were dismantled and examined. The exam-
<br />ination showed the following: (1) The root systems of all plants were princi-
<br />pally located in the pelletized portion of the tailings, and (2) placing the
<br />sewage sludge layer at 3 inches prevented to a great extent the oxidation of
<br />pyrite in the tailings as witnessed by an average tailings pH range of 3.9 to
<br />6.1 and a sludge pH of 6.8 for the 3-inch-deep layer as compared with a tail-
<br />ings pH range of 1.7 to 2.0 and a sludge pH range of 3.6 to 4.0 for the sludge
<br />layer at 15-inch depth. A comparison of figures 3 and 4 shows the difference
<br />in pH change and moisture content within the tailings materials. Proper place-
<br />ment of the sewage sludge layer to control water and gas movement appears excep-
<br />tionally promising for controlling acidification of pyrite-bearing tailings.
<br />Costs of Methods Using Sludge as Pellets
<br />As previously noted, several tested physical, chemical, vegetative, and .
<br />combined vegetative methods have been evaluated as to stabilization effective-
<br />ness, required maintenance, and cost. The costs for the preferred vegetative
<br />methods have ranged from $100 to $1,750 per acre, Based upon the success of
<br />laboratory testing, costs were aiso estimated for the buried sludge layer,
<br />nelletization, and a combination method by the Bureau of Mines. The cost eval-
<br />:ions were made utilizing laboratory data and after consultation with firms
<br />having land-moving, transportation, and pelletizing capabilities. The follow-
<br />ing figures were basic to the Bureau evaluations; Cost of sludge, $1 per ton;
<br />hauling of sludge, $0.42 per. cubic yard, based on a 7-mile haul; amount of
<br />sludge per acre, 130 tons for buried layers and 11 tons for pellets; and a
<br />cost of seeds, fertilizer, and planting of $50 per acre. Calculations on lay-
<br />ing of the buried sludge were made using the following criteria; (1) An agri-
<br />culturally developed method using a broad flat plow for lifting the soil and a
<br />device for spreading a layer of buried material under the long plow, and then
<br />allowing the lifted soil to drop back and bury the layered material; and
<br />(2) use of a conventional wheel scraper and spreader method, The mixture of
<br />pelletizing consisted of, in percent, 92.4 tailing, 6.6 sludge, and 1.0 chemi-
<br />cal binder. Pelletizing was calculated .as being done in place on the tailings
<br />using a recently designed high-speed light-compaction roll pelletizer which
<br />can produce pellets at an estimated cost of $0.80 per ton. All sewage layer
<br />costs were based on application to flat pond areas only, These costs, although
<br />broadly generalized, provide comparison for the different methods and are pre-
<br />sented in table 6.
<br />TABLE 6. - Cost comparison of auxiliary stabilization methods
<br />Approximate
<br />Type of stabilization Effectiveness Maintenance cost per acre,
<br />dollars
<br />Buried sludge layers..... ......
<br />)let cover (1-1 /2 inches)... ..
<br />combined buried sludge plus pellet
<br />cover ............................
<br />Excellent,...) Minimal.... I $405- 810
<br />.....do...... ....do..... 330- 660
<br />.....do......~ ....do..... ~ 735-1,470
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