Description and Cost of Modern Plant - The plant (see Fig. 26) is at the Elephant Butte dam, and is complicated by the inclusion of a plant for the manufacture of sand cement. The description is abstracted from an article which appeared in Engineering Record, October 4,1913.
Fig. 26. General lay-out of construction plant at Elephant Butte dam.
Fig. 27. Sand cement plant - Elephant Butte dam.
(See Fig. 27).
Rock from the sandstone quarry goes to a gyratory crusher and thence, either directly or through a rotary dryer, to a bin above the ball mill. The product of the ball mill goes to the ground rock bins. The cement is received in sacks on cars, the sacks emptied into hoppers and the cement conveyed to a storage bin next to the ground rock bin. From these two bins the ground rock and the cement are taken via screw conveyors to an automatic mixer which can be set for any desired proportion; thence by a mixing conveyor to a long conveyor delivering to four hoppers which feed the four tube mills where the final grinding is done. The product of the four tube mills goes onto a conveyor which delivers onto an inclined belt conveyor leading to the top of the storage bins. The cost of the sand cement plant, including machinery, buildings, erection and overhead charges was about $56,000,
Fig. 28. Crushing plant - Elephant Butte dam.
(See Fig. 28).
This consists of two No. 7 1/2 Symes gyratory crushers, elevators, screens and bins. Each crusher has a capacity of 100 tons to 150 tons per hour and, with its elevator, requires a 100-h.p. motor. Trains of 6-cu. yd. cars dump onto the crusher platform 20 ft. below the track. Gates, operated by plungers underneath, regulate the delivery to the crushers. One crusher discharges direct to an inclined bucket elevator, the other to a short platform conveyor which discharges into a vertical bucket elevator. The two elevators deliver to screens which separate the sand and stone, delivering them to their respective bins. A house was erected over the crushers. Two men are required for each crusher. The total cost of this crushing plant, including bins completely installed ready for operation, was $31,500. The depreciation cost charged against crushing rock, on the assumption that the plant is scrapped when the job is finished, is 10.6 cents per cu. yd. The labor cost of production is approximately 12 cents per cu. yd.
Elevation of Crusher and Mixer Plant and Conveyors.
Fig. 29. Mixing plant - Elephant Butte dam.
(See Fig. 29).
Three 80-cu. ft. Milwaukee mixers discharge their product into hoppers holding two or three batches. This practice is followed in order that the operation of the mixer may not be interrupted by the non-arrival of the skip, and (which is as important) that the travel of the skip need not wait on the operation of the mixer. Above each mixer are three measuring hoppers like truncated pyramids connected by a rectangular section which telescopes to change the capacity. An automatic hydraulic-ally operated interlocking mechanism opens and closes the gates (a) from bins to measuring hoppers, (b) from measuring hoppers to receiving hopper and (c) from receiving hopper to mixer. The amount of water per batch is determined by the setting of a siphon attachment to the supply tank.
Three tracks run from the mixing plant at right angles to and beyond the line of the cableways. At the opposite end of the tracks is a three-drum hoist handling a car on each track. With this arrangement it is possible for each mixer to be operated by one man though two seem to be necessary to secure uniformity of product. The total cost of the mixing plant, including all machinery, storage bins, elevators (not including crushing plant) and haulage system, is $57,800. The depreciation cost is estimated as 13 cents per cu. yd. The operating labor cost at full capacity will be 8 cents per cu. yd., although operation to date, at a small fraction of the capacity, has cost 14 cents.