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Placer mining, also called alluvial mining by the British, is an aqueous extraction method intended for recovery of heavy minerals from placer deposits using water to excavate and transport the mineral. Placer deposits consist of some valuable mineral that accumulates in weathered rock or overburden (eluvial placers), in stream sediments (alluvial placers), or in beach deposits (beach placers) as a result of natural weathering and erosion processes. Glacial till and moraine placers are poorly sorted until subjected to stream action. Residual placers are mineralized rock weathered and in place principally through chemical change. Stream or fluvial placers are formed by running water that carries the lighter materials away faster than the heavier minerals, thus concentrating them. Historically, fluvial placers have been the most important. The higher specific gravity minerals concentrate under placer- forming conditions. The process creates minerals that are tough and chemically inert. Typical minerals recovered by placer mining are gold, platinum, tin, diamond, titaniferous and ferrous iron sands and gemstones. Exploration Methods Sampling and Prospecting- In reconnaissance sampling, there is indication of something of interest. Because of their complexity, placer deposits are the most difficult mineral deposits to sample. Hints On Sampling
Where shallow unconsolidated deposits are
under water or have a high water table, any dewatered excavation
below the water table will have inflowing water that carries
the fine material preferentially. Nested caissons would be necessary
to avoid contaminated samples caused by the inflow of waterborne
fine-grained materials. In these nested caissons, some central
portion of the pit or shaft can be protected by deeper exploration. Hand and mechanical churn drills are most commonly used after the pit and trench methods. In churn drilling, an outer casing of pipe led by a hardened drive shoe is driven down a few inches in the placer, water is poured in, and the loose materials in the bottom of the casing is slurried and bailed out either by a pump or bail consisting of a tube with a flap valve in its lower end. The bail is lowered to measure the core rise. The core rise is the height between the bottom of the drill shoe and the material in the bottom of the bail. In placer gold or platinum exploration, the volume of the sample is calculated from the theoretical volume of the drill hole based on the diameter of the cutting shoe. Once the core rise has been determined the bail is raised and lowered quickly to churn up the material on the bottom and forcing the loosened material in past the bail's flap valve. The bail material is dumped into measuring containers at the surface and the volume of the sample is measured again. When the bail has excavated material as
close to the bottom of the drill shoe as the desired, the casing
is driven by hammering down or rotating it or a combination of
the two, depending on the type of drill. A few inches of plug
are left in the bottom of the casing to prevent free inflow.
The cycle of drilling and bailing is continued to bedrock. Reverse circulation drills- have been used for placer sampling in number of instances, but with varying degrees of success. The drills use a double walled casing, with the opening between the inner and outer wall carrying either air or water under pressure downward tot he cutting shoe. Here it aids in cutting loose the material at the bottom of the hole and then carries it to the surface for recovery. The larger diameter, 8 to 12, reverse circulation drills give much better samples. Sonic drilling- In shallow deposits, high-powered vibrations in high frequencies are transmitted down the casing to a cutting shoe. This method seems to work better in the clay types of materials. Placer exploration field logs must contain all the measurements that are taken in the field. The results of calculations should be entered separately from measurements. If changes are made, make sure they are explained clearly. Placer Mining Methods Drift mining-
This method is often used with minerals in relatively thin horizons
that precludes excavation from the surface. The zone of concentration
may extend into bedrock or on a false bedrock, often clay, or
be in an intermediate horizon of gravel. Typically this has been
used to mine gold and platinum. This method was largely responsible
for the recovery of high grade gold in the Klondike. valued at
250 million. 75% of this was taken in the first ten years by
drift miners.
Large-scale, dry-land mining methods must have adequate water available for mineral processing. although many waterless rougher recovery processes for heavy mineral have been tried , the economic success rate is very low.
Until the advent of dependable backhoe excavators, dragline excavators were most commonly used to feed floating washing plants. Where the the material to be mined is not suited to a backhoe, the draglines are still used. The dragline method does not have the control or power that backhoe buckets do. The floating plant must be designed to coordinate with the excavator. The excavator must be able to elevate its bucket high enough to dump cleanly into the washing plant's hopper. Backhoes used with washing plants are usually limited to a shallower digging depth than their maximum in order to clean up bedrock over a reasonable working radius. Bucketline dredges are capable of continuous excavation with mineral separation plants aboard, are very y efficient mining machines. they mine ore, process it, and discard tailing to waste in continuous stream. The process of dredge mining suffers from the in ability to stockpile material during malfunction of any piece of machinery in that sequence. As a result, a mineral dredge must be overdesigned in comparison with other mining plants, but there are almost none of the transportation problems of other mining methods. Ore flows from the mine face to the tailings area in an unbroken stream. On a bucketline dredge, the stream often moves through the system under the force of gravity once it was dumped from the buckets. Bucketline dredges are capable of high excavation rates in the hardest alluvial ground dredgeable because they are capable of transmitting to the mine face the greatest amount of digging force of any kind of dredge.
Surface mining can be subdivided into various classes and subclasses.
The advantages and disadvantages of one type of of surface mining vs. another is related to the types of equipment utilized and the associated costs and benefits derived from the use of equipment. Strip mining has the greatest choice of equipment, particularly for mining the flat lying coal seams in gentle topography. Open pit mining is usually limited to shovel/truck or front end loader/truck type operations. shovel /rail type operations of the past have been almost phased out in favor of truck haulage. References Resources Links for Mining Equipment and supplies
Alaska Mining and Dive Supply-links to supplies for mining
supplies and equipment http://www.akmining.com/mine/minclass.htm Alaska Placer Mining Applications Field Photos for Placer Mining -Some pictures
of a sluice box also. |
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