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Sluicing

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Mining Processes

Sluicing, used in placer mining, is the process of a using a sluice box to separate the gold from gravel and overburden found in placer deposits situated in streams, rivers, or creeks. Sluicing typically follows panning. Panning is used to gather a sample from a placer deposit and sluicing is used to process larger amounts of gravel for maximum gold recovery. Sluicing, on average, can yield 10 to 200 times more material than that recovered by panning.[1]

Sluicing in its simplest form is accomplished by shoveling material and gravel directly from the bottom of the river or stream into the sluice box. A sluice box placed on an incline functions as a trough or channel. As water is directed into the box, the gold gets trapped in the riffles affixed to the bottom of the box. Since gold is heavy, it sinks into the riffles while the remaining gravel and larger-sized rocks are washed across the channel or trough.

Sluices range in size from miniature, meant for individual operation, to medium-sized, used with suction dredges, to large-scale sluices fueled by heavy equipment like bulldozers.[2] Sluices can be fabricated from a variety of material from like wood, metal, or plastic.

Contents

[edit] History

Sluices were first used in the alluvial mining of gold placer deposits during the Gold Rush. Sluices were developed as extended versions of Long Toms. It was common to group a number of sluice boxes together in a long line with large crews of miners working the system. This practice quickly evolved into a variant of ground sluicing. Sometimes ditches were dug to facilitate the movement of water diverted from a nearby river to soften gold and gravel. The dirt and gravel would be loosened manually by picks and moving water. Gravity also helped to push the material down the sluice box. The gold was then removed from the sluice box and panned.[3]

[edit] Process

Water channeled through a sluice box is used to separate gold from gravel retrieved from a placer deposit.
In a typical sluicing operation, the sluice box is located along the river or creek at a proper depth so the flow of water can be directed to the box. Once the sluice box is set up, gold-bearing material from the bottom of the creek or river is shoveled and fed into the sluice box. Sometimes the material passes through a screen called a grizzly to remove larger-sized rocks too big to pass through the sluice. A grizzly is used when there is a limited amount of water. Screening the material not only removes unnecessary larger-sized material, it also improves gold recovery since less water velocity is needed in the recovery phase of operation. Screening material in this way is called classification, and the materials passing through the screen are called classification materials.[4]

Water supply is the most important aspect of a sluicing operation. If there is not sufficient water or water flow for flushing, pumps, pipelines, or even dams with special gates to contain the water may be used. For example, ground sluicing, the process of harnessing the cascading or running effect of water to break down gravel, requires a greater supply of water. As an application, ground sluicing is meant to streambed deposits.[5] The use of pipelines, flumes, or ditches is required if sluicing is carried out higher up on a riverbank or terrace.

Sometimes water is pumped or siphoned from a dam, reservoir, or pool created artificially to compensate for inadequate water supply. Water is stored and then released against or across the materials intermittently. This process, known as booming, is considered a variant of sluicing.[6] Water is held back and accumulates in a dam or reservoir controlled and contained with a gate mechanism. A quick release mechanism releases the impounded water for a maximum washing effect of the material.[7]

[edit] Highbanking

Pumping or siphoning water supply to a sluice box requires a motorized sluice, also called a hydraulic concentrator. In mining, the use of motorized sluicing is referred to as highbanking or hibanking. Highbanking involves using the water flow of the creek or river to keep the gravel passing and moving through the riffles in the sluice.

Motorized sluicing has certain advantages over normal sluicing. For example, the supply of water can be located several hundred feet away from where digging occurs. A pump and engine assembly is used to pump water from the water source through a hose to the sluice, directly mimicking natural water flow. The pumps are either electric or gasoline powered and some units, called re-circulating highbankers, are even designed to re-circulate the water supply.[8] Re-circulating hibankers are usually operated in desert areas where there is very little water.

Some motorized sluices also come equipped with a hopper box that features spray bars and a grizzly. The high-pressure spray washers are used to remove clay and debris and may feature a water valve to control the flow of water through the sluice. The screen and sluice components can easily be set up at the work site, meaning payload is shoveled directly into the screening section without having to be hauled. Motorized sluices also come with adjustable leg supports for fast assembly, making them easy to operate on almost any type of terrain.[9]

[edit] References

  1. Sluicing for Gold. E-Gold Prospecting. 2008-11-27.
  2. Sluicing for Gold. E-Gold Prospecting. 2008-11-27.
  3. Sluice Boxes. Sierra Nevada Virtual Museum. 2008-11-28.
  4. Classification of Material in Sluicing. Gold and Treasure Hunter Magazine. May/Jun 1993. 2008-11-27.
  5. Small Mining. Sluice. Alluvial Exploration and Mining. 2008-11-27.
  6. Small Mining. Sluice. Alluvial Exploration and Mining. 2008-11-27.
  7. Classification of Material in Sluicing. Gold and Treasure Hunter Magazine. May/Jun 1993. 2008-11-27.
  8. Highbankers - Power Sluices. Gold Fever Prospecting. 2008-11-27.
  9. Highbankers - Power Sluices. Gold Fever Prospecting. 2008-11-27.