Montana oil and mining journal. [volume] (Great Falls, Mont.) 1931-1953, 1939-11-04
continued from last weekly

the many different types of small
nlants would be impossible mere
The following description shows
what the average doodlebug Is ike
in construction and operation.
The average doodlebug is adap-
ted IN fairly shallow ground vary
ing from to 20 feet in depth with
a soft bedrock or hard-pan. Wheth-
er the dredge is Of the floating
or dry-land type, the operation IS
somewhat the same. The eXcaVator'
generally mounted on caterplller
freads. transverses the bank IN
front of the washer and delivers
the excayated material to the mop
ner. To save time in spotting the
bucket over the hopper the hopper
sbould be of generous size at least
10 feet. or better, II feet. This in
turn means that the hopper bottom
will not have enough slope for the
material to slide readily down to
the screen. Many of the floating
dredges depend upon the rocking
bottom of the dredge caused by
the depositing of the material IN
Tn the hopper to slide the material
into the screens. Many forms OF
mechanical feeders have been de-
veloped by operators, but IN the
long run they return to the gen
erai tyne of hopper. The usual ten
dency is to keep several streams OF
water shooting with fair force into
the hopper and washing the mater
ial into the screen. Some opera
tors have kept an attendant on
the hopper with a large hose TO
wash the material into the screen
The hoppers should be OF fairly
heavy construction, well supported
and reinforced to stand the con-
tinual shock from the dropping OF
tons of gravel into the hopper
A revolving screen four or five
feet in diameter and at least 10
feet in length of actual screening
surface and preferably perforated
with 3-S to 7-16 inch round holes
will be required for a capacity OF
1000 to $500 cubic yards per day
OF the average placer ground. In
placer dredging the operator should
be thoroughly acquainted with the
ground which he is working. The
percentage of fines in the ground
should be known so that the type
and size of the screen be estimated
fairly we'll. Direct gears and chain
drive on revolving screens are not
too successful and should be avoid
ed
The best type of screen now be-
ing used has heavy tread rings OF
cast steel near each end, supported
on two pairs OF heavy. large dia-
meter cast steel rollers. The pair
of rollers at the lower end OF the
screen are powered by bevel gear
ing. which transmits the power to
the drillers on the screen. The grade
on the screen varies from q to
21t inches per foot The steep grade
OF % inches per foot may be used
on material that is free and sandy
and does not require much wash
ing to put the fines through the
screen.
An eight-inch centrifugal pump
of good type and design, running
at a speed which will give so to 70
feet of lift will furnish water for
an 1000 to $500 cubic yard plant.
The dredge should also be fur
nished with a two or thFee-inch
Dump for pumping, cleanup. fire
protection and cleanings of the
decks. Where the dredge is float
ing small hand pump of some
tyne should be kept for hull drain
age. -.
diameter screen. a 24-inch belt con-
veyor running at about 800 feet
ner minute, mounted on light
hut substantial steel truss frame
work. is sufficient for the stacking
of the tailings. During the past 30
years of dredging rubber belts have
been developed which are really
excellent for the removal of tall
ings and giving very long service.
They seem very high priced to
those new in the business of dredg-
ing. but those with experience in
this field are glad to pay the price
and get the goods and service.
The proper screening Of the
gravel is the first very important
requisite for the proper saving Of
the values contained. Since it takes
more water to wash the gravel in
the screen than 1t would over the
gold saving tables many prefer to
add all necessary water for washing
through the nozzles which wash the
gravel In the screen. Feeders with
adjustable valves should be provid-
ed so that additional water may be
added to the gravel passing over
the tables when required. 1t IS al
so very essential that the pulp be
uniformly distributed over the
tables. This distribution of the
pulp over the tables has often been
the difference between the success
and the failure of an operation.
Many of the rifles used in the
smaller dredges are direct copies