Table of Contents
The design and specifications submitted in this paper are not offered as satisfying all conditions of gold dredging. Many rich stream-beds would not give “ elbow-room ” for such a dredge: many otherwise favorable gold-bearing areas are too small to warrant the cost of its installation; and, in some localities, as I have already remarked, the supply of adequate power might be, at the present time, a matter of economically insuperable difficulty. But I believe that, under suitable conditions, the use of hydraulic suction-dredges of a minimum capacity of 1,000 cu. yd. per hr. would be exceptionally profitable; and I hope that my suggestions will elicit the discussion by engineers in this department of a proposition which, to my mind, promises so great an advance in this relatively new form of mining industry.
Several manufacturing concerns have been mentioned, as a matter of record, in the foregoing pages; but I need hardly say that these references are not intended either to advertise particular parties, or to convey the impression that these parties only would be able to furnish the machinery contemplated. On the contrary, there is nothing in the design here proposed which could not be constructed by any establishment experienced in such work.
Power
The maximum power-requirements of a gold-dredge are estimated from the Steers dredge in operation, and proportioned according to the Oroville practice with dredges having a nominal capacity of 100 cu. yd. per hr. On this basis, the power-requirement would be:
Pump
In the opinion of Carl Lager, Superintendent of the Morris Machine Works, Baldwinsville, N. Y., which I adopt, with appreciation of his competent advice, a dredge intended to handle continuously gravel that would enter a 24-in. pipe would require a pump of cast steel, heavily reinforced and provided with manganese- or chrome-steel liners. The shell would need to be about 12 ft. in diameter by 4 ft. in width. The runner would be 7 ft. in diameter and make 170 rev. per min. The runner-shaft would have a slip-coupling, so that the runner might be stopped short by an obstruction without anything breaking. For dredging to a depth of 60 ft. the economical velocity of flow in a 24-in. pipe would be 10 ft. per sec., giving a flow of 14,200 gal. per min. If 23 per cent, of this flow was gravel, it would give the required 1,000 cu. yd. per hour. In practice, a skillful lever-man, working in free gravel, should raise 25 per cent, continuously. The outboard suction-pipe would be about 90 ft. long. For the less than 200 ft. of pipe required, the friction-head, at the estimated velocity of 10 ft. per sec, would, apparently, be less than 10 ft.; but, for safety, the total friction-head was assumed at 60 ft. and the pump-efficiency at 40 per cent., giving an estimated power-requirement of 600 h.p. delivered on the runner-shaft. The pump would discharge at one side of the bottom, and the discharge-pipe, before curving to the sluices, would have a removable section for taking out gravel in the event of a shut-down under load. The pump would weigh about 65,000 lb., and cost not more than $10,000. For a reserve set of liners, $2,000 more may be allowed; and for an extra runner, $1,500.
Cutter
The cutter, to provide against breaking when stalled, should be driven by a 9-in. chrome-steel shaft, which should be supported by the trussed suction-pipe. Cast-steel pipe would probably be best for continuous gravel-suction. The suction-pipe and cutter, with drive and gears, would weigh about 70,000 lb., and cost about $9,000. The curve of the cutter-blades should be such as to leave the mouth of the pipe but a few inches above bed-rock when dredging at the full depth of 60 ft. The diameter of the cutter should be, say, 7 feet.
The Steers dredge has two large timbers as stiffeners to the suction-pipe and cutter-shaft, as shown in Fig. 1. It would be well to use, instead of these, two riveted hydraulic pipes, 24 in. in diameter, which would give a net buoyant effect of about
326 lb. per running foot, with the further advantage that the lengths of supporting pipe could be made exactly equal to the lengths of the cutter-shaft and suction-pipe, and, when desired, the whole ladder could be materially shortened or lengthened. These pipes and the cutter-shaft could be carried with advantage in a lattice truss, with a removable section, to permit permanent use at shallow depths.
Winch
Owing to the difficulty on the Steers dredge of throwing the cutter in and out of gear, it seems advisable to have the winches entirely separate. They should be very powerful machines, and, for gold-dredging in flowing rivers, would be found more satisfactory if divided into two units, one on each side of the dredge, each with a spool projecting, for warping, handling lines, etc. There should be 12 winch-barrels, of sizes proportioned to their respective services: two for headlines (one in reserve), which would be built to carry each 2,000 ft. of heavy steel cables; one for ladder-hoist; three for operating a snag-and-rock grab ; two for swinging-lines; two for stern-lines; and two for the spuds.
The winches should have gearing for two speeds: a very slow one for difficult situations, and a relatively fast one for ordinary circumstances. Extra lines would be an important part of this equipment. The total for this winch may be estimated at about $50,000, and a weight of 100,000 lb. should be allowed.
Bank-Pump and Clean-Up
It will frequently be found desirable to cave the bank ahead of the dredge. The nozzle could be attached to the snag- boom, and handled close to the bank. The same pump would also answer as a primer, and to supply the amalgam-barrels and clean-up tanks; 50 h.p. would fill this requirement. A weight of 10,000 lb. and a cost of about $1,500 should be allowed for this item.
Force-Pump
The force-pump to supply water under pressure to the main runner-bearing in the dredging-pump, would have to be running both before and after the dredging-pump was started, as well as during dredging; 10 h.p. would cover this item, with, about 700 lb. weight and a cost of $600.
Electric Light
The dredge should be lighted throughout by electricity, with incandescent lamps handy to the machinery, two arcs at the bow, two arcs over tables, and one in the engine-room. Flaming-arc lamps should be used. A searchlight should also be placed over the pilot-house, to be switched in when desired; 15 h.p. should cover this item, with a cost of $1,000 and a weight of 3,000 pounds.
Maintenance Shop
In connection with a large forge and blacksmith equipment, the dredge should have a lathe, shears, radial drill, and milling-machine ; also a traveler for handling the plates that would have to be constantly ready for the screen. It would be advisable to do this work on board, at least until the screen-requirements of a given district were standardized; 10 h.p. and a cost of $3,000, with a weight of 10,000 lb., should cover this item.
Stacker
The capacity of the stacker should be such as to be able to handle a maximum of 10 cu. yd. per min.; length, say, 89 ft. A belt, running at not over 300 ft. per min., would seem to be the best to handle the gravel, without bringing too heavy a weight on the stern. A main belt, 60 in. wide, with the sides gently curved by idlers for 18 in. from the edges, and a superimposed flat belt to take the wear, should answer the purpose ; 90 h.p., a weight of 40,000 lb., and a cost of $10,000, should cover this feature.
Screen
In view of the screen-areas of present dredges, and the record of the Sweetser-Burroughs dredge, quoted above, a screen with a surface of 2,000 sq. ft. would apparently be able to handle 1,000 cu. yd. per hr. The pipe would discharge, at 26 to 30 ft. above the water-level, into a stationary sluice, 25 by 20 ft. in size, with a bottom of 0.25-in. plate-steel, set at a grade of 0.25 in. to the foot and perforated with 1/8-in. holes, countersunk on the under side. This stationary screen would discharge, in turn, on to a shaking-screen of the same width extending over the rest of the length of the tables. The holes would increase in diameter to 0.5 in. at the lower end of the screen, and should be close enough together to make sure that all the water will drain through them before the gravel reaches the lower end. The shaking-screen would be set at a grade of 1.5 in. per ft., with an adjustable hanger for reducing the grade as desired, as shown in Fig. 2.
Gold Saving Areas
Dredges now in use have a saving-area of from 750 to 1,000 sq. ft. for a capacity of 100 cu. yd. of gravel per hour. This ratio should be at least maintained, and increased as much as practicable. It seems quite feasible to get a satisfactory area. The old Plutus and California, the first dredges in Oroville to use the shaking-screen (both steam-shovel dredges and long since abandoned), used a high drop of 2 ft. or more on to the tables, with the idea that the gold that fell with this impact would stay and amalgamate. But Vail, a veteran gold-saver, argued that the gold which would stay by reason of its weight would stay anyhow, and that by utilizing this space for a riffle-board, having shallow auger-holes, he could provide a greater area to amalgamate the fine gold. His hoards, set zig-zag from the upper to the lower level, were successful; and, somewhat modified, they are used in some of the latest dredges. The following is the suggested gold-saving area for the dredge here proposed :
Hull
For a field where the dredge would never have to move out of the reach of commercial electricity, and would not be compelled to carry long lines to work or warp against strong currents, the pump could be set in the center of the dredge, a much shorter hull could be used, and greater table-area secured by increased width. But for use in foreign river-bottoms, a hull 175 by 60 by 8 ft. would be better. The bow should be tapered and have a gentle rise for, say, 20 ft., and the stern should have an equal curve for the “ get-away.” The importance of the stern curve is sometimes not fully understood. Church has clearly shown that a blunt stern on a scow is a greater strain on a tow-line than a blunt bow, on account of the great mass of water, the momentum of which it has to overcome.
Properly constructed of steel, with suitable wood-lined quarters for manager, captain, and crew, and adequate strength in every part for a sea-voyage, as well as for the strain of regular operation, the hull ought not to cost more than $45,000.
Fuel
In New Zealand and elsewhere, dredges are operated with steam, using coal as fuel. But gold-dredges in South America would have to depend, at least for some time to come, on wood as fuel. Those who have been compelled to make steam with comparatively fresh-cut wood, wet by rain or by transportation in canoes, will probably agree with me that it is unsafe to estimate on maintaining 120 h.p. on less than 1/3 cord per hr., or 8 cords per 24 hr. At this rate, 1,200 h.p. for the proposed dredge would require 80 cords per day, which, under ordinary circumstances, would be a prohibitory condition.
The paper of Mr. Langton on The Power Plant of the Moctezuma Copper Co. at Nacozari, Sonora, Mexico, raises the interesting question of the use of gas-engines driven with gas from wood.
I am inclined to think that, under suitable circumstances, this means of generating power might be found practicable and profitable in some regions such as I have mentioned. But the source of power is a question of locality. In many places it would be best to utilize natural water-power through the electric current. For the proposed dredge, when electricity is not available, I suggest a marine producer-gas plant, as shown in Fig. 2, which, according to the Loomis-Pettibone Engineering Co., will deliver the required power with a wood-consumption of only 17 cords per day. The cost of such a plant would be about $12,000.
Engines
Three gas-engines would be required: one for the dredging- pump, with four 20- by 25-in. cylinders, guaranteed to develop 700 h.p. at 170 rev. per min., and able to stand up under 1,000 h.p., though with somewhat less economy. This engine, including all auxiliaries, air-compressor, circulating-pump, etc., would weigh 40,000 lb., and cost $30,800.
The dynamo to distribute power to the various motors would be run by an engine with two cylinders (of the same dimensions, so that one repair set only need be kept), weighing 20,000 lb., and costing $15,400. This cost is stated to cover an extra cylinder set, extra one-half crank-shaft of total weight of 6,500 lb., and a 14 h.p. donkey-engine of two cylinders, with dynamo for lighting and pump, as well as a compressor for filling the air-tank for starting the large engines.
Crew and Expense
For continuous operation and the occasional handling of heavy lines, the following crew should fill the requirements at the stated daily wage, plus food :
The allowance for food and wood is liberal, and a smaller crew would perhaps suffice; but these figures should not be cut for preliminary estimates.
Capacity and Cost Per Yard
With ordinary care in painting and keeping up, a dredge built in the manner suggested should last many years. With all the machinery of the best character, a full supply of duplicates for all possible contingencies, a good crew and good management, the shut-downs should be but slight, outside of encountering snags and sunken logs, to handle which this hydraulic dredge would be as well equipped as any other. Very few stones would be found in the average dredging-field that would not go through a 24-in. ring, so there should be little time lost on this score. It seems entirely reasonable to expect to dredge 20 hr. per day. This, at 1,000 yd. per hr., would give a daily capacity of 20,000 cu. yd. of gravel.
Notwithstanding the smaller sum of the component items of cost given above, it would not be prudent to reckon that the cost of such a dredge, by the time it has been set up in the port of its construction, given a trial run, then housed and towed to its destination, and put at work on the ground, would be less than from $200,000 to $250,000. It should be remembered, however, that a dredge of this capacity and of the table- area to be used, and to employ commercial electricity as power, would cost less than half this sum, or no more than a good 7-ft. chain-bucket dredge.
A sinking fund, to retire $200,000 in 10 years, would require $83.29 per $1,000 per year, or per day (at 300 days per year), $55.53. This, added to the daily running-expense of $254, with a further allowance of $25 for general expense, gives $335 as the daily grand total; and this sum, divided by 20,000 cu. yd. of gravel handled daily, gives $0.01675 as the cost per cubic yard.
Even if the ground should be unusually full of logs, it is evident that considerable time could be lost without making the cost per yard excessive. In fact, the power of the dredge is so calculated that more than 1,250 cu. yd. of free gravel could be readily dredged and screened per hour, so that the assumed average of 1,000 cu. yd. per hr. can easily be maintained.