ver,
Colo., uses a continuous blast of air from a fan. As usually
constructed, each unit consists of a troughlike box, semicircular in
vertical section and covered by a perforated plate, somewhat less than
an inch thick. The perforations are semicircular in cross section and
about an inch in diameter. They are partly closed beneath by wire
gauze, which allows the air blast to pass but retains the concentrate.
The boxes, which are usually set in pairs on a frame, have a gentle
inclination and receive lateral agitation from a cam. The pay dirt is
fed upon the upper end of the plate and, as it passes down the slope,
the fine material is blown away and the concentrate is caught in the
holes or recesses which serve as riffles, the tailing escaping at the
end. To clean up, the machine is stopped and the plate is rotated on a
longitudinal axis, dumping the concentrate into the trough below, from
whicn it is removed. * * * Another type is the Shumway concentrator. *
* * This consists of a shallow wooden box covered with thin canvas
through which a continuous air blast is forced by a fan. Over the
canvas, wire netting of about half-inch mesh is laid. By means of cams
the box, which is slightly inclined, is agitated laterally and the dirt
fed upon the canvas, at the top of the incline, passes down the slope,
leaving the concentrate in the meshes of the wire, wnile the tailing
falls to the ground. To recover the concentrate, the operations of
this machine must be suspended and the table tilted for cleaning up. *
* * The Jardine concentrator, made in San Francisco, is the result of a
diligent effort to construct a durable machine which is continuous in
operation and does not stop for cleaning up. The air blast comes from a
fan, but is made intermittent by a rotary valve. The shallow box which
serves as an air chamber is interrupted across its length by riffles
of parallel metal plates with intervening slits which guide the
concentrate down into cylinders in which worm conveyors rotate,
conducting the gold and heavier materials to a trough at one side.
Between the riffles, the surface of the table is formed of a
cotton-cloth screen, through which the air blast passes. Lateral
agitation is provided and, as the dirt passes down the sloping table,
the fine light material is blown away. The concentrate caught by the
riffles is delivered into the trough at the side ana the tailing
passes away at the end of the table.
*
* * The Stebbins table, made in Los Angeles, in general form and
action resembles closely the Wilfley table. The surface and the
riffles are formed from sheet metal, the latter being about 1 inch
apart. Air is introduced beneath the table in a continuous blast from
a fan and reaches the bed of dirt through minute perforations in the
metal surface of the table. These perforations, which are as close
together as possible, are rectangular and about one-fourth inch long
and one-fortieth to one sixty-fourth inch wide. The longer axes of
these slits stand at about 45° to the direction of the riffles. The
form of the slit gives the air jet an inclination of about 10° above
the surface of the table, and the air sheet formed by these jets flows
with the dirt in its descent along the slope. * * * The total number of
machines devised for this work can probably be reckoned by hundreds.
Most of them have been short lived.
*
* * The concentration, to be successful, must be capable of
working effectively on material of all sizes up to one-eighth inch.
This mesh has been determined for many localities in Sonoro, Mexico, as
one which will allow most of the coarse gold to pass through with the
fine. Should there be much coarse gold present in the gravel, the
screen must be made proportionately coarser and the concentrator must
work on coarser material. Primarily the pay dirt treated must be dry. *
* * The cemented gravels are crushed with great efficiency by the
Quenner pulverizer, which consists of a revolving trommel or barrel of
steel staves with a shaft rotating independently within and carrying
chain hammers. The hammers effectively break up the calcareous cement
and leave the cobblestones and pebbles with nearly clean surfaces to be
ejected at one end of the barrel, while the pulverized cement escapes
through the quarter-inch spaces between the staves. Thus in many
localities the material subjected to dry concentration for the recovery
of the gold is only about one-half of the total mined.
Plomosa district.—The Plomosa mining district,1 lying east of Colorado River in Yuma County, Ariz., is situated—
in
the Posas Valley, a great north and south depression, with the Plomosa
Mountains forming the eastern border and northward extension of the
Castle Dome Range on the west. Posas Valley trends northward 30 or 40
miles, and is from 10 to 15 miles wide. It slopes to the north and
affords a very extensive field. * * * In this valley are situated * * *
on its eastern side the Plomosa placer and on the western side an
extended deposit of gold-bearing gravel known as the La Cholla, Oro
Fino, and Middle Camp. In some localities pits have been sunk to a
depth of 20, 30, and 50 feet or more to beds of cement, which are
richer than the gravel. Near the moun-
1 Church, John A., Plomosa mining district—extracts from a professional report dated Mar. 19 1901. 19373°—m k 1912—vol 1------17
