A drywasher is a common
desert
A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...
mining
Mining is the extraction of valuable minerals or other geological materials from the Earth, usually from an ore body, lode, vein, seam, reef, or placer deposit. The exploitation of these deposits for raw material is based on the economic via ...
tool for
gold mining
Gold mining is the extraction of gold resources by mining. Historically, mining gold from alluvial deposits used manual separation processes, such as gold panning. However, with the expansion of gold mining to ores that are not on the surface ...
.
A drywasher is like a
highbanker, since it uses a motor and a form of
sluice
Sluice ( ) is a word for a channel controlled at its head by a movable gate which is called a sluice gate. A sluice gate is traditionally a wood or metal barrier sliding in grooves that are set in the sides of the waterway and can be considered ...
, but it has no need for water. It drywasher operates by the use of air.
By forcing air to flow up through the material as it moves down the sluice, the heavier materials, like
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
, will stay at the bottom and get trapped by the
riffles
A riffle is a shallow landform in a flowing channel. Colloquially, it is a shallow place in a river where water flows quickly past rocks. However, in geology a riffle has specific characteristics.
Topographic, sedimentary and hydraulic indica ...
while the lighter materials will flow up and over the riffles or be blown away. The lighter material that is not blown away will slowly flow down the sluice and fall out the end of the sluice.
Drywasher - A mechanical apparatus used for separating free particles of
placer gold from dry
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
s. Dry placers represent a major portion of all
alluvial
Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...
gold deposits around the globe. Extreme
arid
A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most ar ...
ity in southern hemispheric regions of the planet causes a slower rate of
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
on placer deposits than if rainfall were plentiful and
regional watersheds were
hydrologically
Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is call ...
active. This absence of water directly correlates to the low moisture content of alluvial sediments and the short distance gold moves from its hardrock source. A drywasher works on the principle of
gravity separation
Gravity separation is an industrial method of separating two components, either a suspension, or dry granular mixture where separating the components with gravity is sufficiently practical: i.e. the components of the mixture have different specific ...
. Since gold, at 19.1
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water (molecule), wa ...
, is much heavier than most other minerals found in local sediments, a
facsimile
A facsimile (from Latin ''fac simile'', "to make alike") is a copy or reproduction of an old book, manuscript, map, Old master print, art print, or other item of historical value that is as true to the original source as possible. It differs from ...
of certain natural conditions (which would involve the use of water, if it were available) can be mechanically reproduced with a simple machine using dry air instead of water as the medium of separation. The process of excavating dry, compacted sediments will often loosen or unpack them. Then, as long as the material is sufficiently dry, these
gravel
Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone.
Gravel is classifi ...
s can be processed with a drywasher in a relatively efficient manner.
A drywasher was also known, at one time, as a Mexican air jig. To use a drywasher, first, one must determine if the raw material is dry enough to be efficiently processed. Once the machine is set up and stabilized, using a shovel, the
dirt
Dirt is an unclean matter, especially when in contact with a person's clothes, skin, or possessions. In such cases, they are said to become dirty.
Common types of dirt include:
* Debris: scattered pieces of waste or remains
* Dust: a genera ...
,
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
,
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...
, gravel, etc. is loaded onto the uppermost part of the device — the screen, or "grizzly". Sediments classified by the grizzly, drop into another part of the
apparatus known as "the
hopper
Hopper or hoppers may refer to:
Places
*Hopper, Illinois
* Hopper, West Virginia
* Hopper, a mountain and valley in the Hunza–Nagar District of Pakistan
* Hopper (crater), a crater on Mercury
People with the name
* Hopper (surname)
* Grace H ...
". Once the machine is activated, the finer, gold-bearing material travels down an inclined, vibrating chute, passing over a series of "riffles" in a recovery tray. This process varies slightly from machine to machine. Heavier minerals, principally gold and fine-grained black sands (various
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
minerals), are separated from the paydirt and reconcentrated by this means.
Panning, or some other final separation process, comes afterwards.
The first machines were, in all likelihood, operated manually. A simple one can be constructed using a wooden framework, a hopper, a recovery tray,
bellows
A bellows or pair of bellows is a device constructed to furnish a strong blast of air. The simplest type consists of a flexible bag comprising a pair of rigid boards with handles joined by flexible leather sides enclosing an approximately airtigh ...
,
lever
A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or ''fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is div ...
, and cord. Before the advent of modern metal
screening
Screening may refer to:
* Screening cultures, a type a medical test that is done to find an infection
* Screening (economics), a strategy of combating adverse selection (includes sorting resumes to select employees)
* Screening (environmental), a ...
material, sorting of gravels into smaller particles might have been done by hand. Such elimination of larger rocks aids in separating gold from dry,
auriferous
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic luster and pale brass-yellow hue giv ...
sediments to be processed, consequently, any modern "puffer-belly" could be outfitted with a screen, or "grizzly" for sorting the dry gravels into two sizes.
References
*.
*. Date unknown. Republished in electronic format with foreword by th
''ICMJ's Prospecting and Mining Journal''
{{Extractive metallurgy
Gold mining