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Hydraulic mining is a form of mining that uses high-pressure jets of water to dislodge rock material or move sediment.Paul W. Thrush, A Dictionary of Mining, Mineral, and Related Terms, US Bureau of Mines, 1968, p.560. In the placer mining of gold or tin, the resulting water-sediment slurry is directed through to remove the gold or tin. It is also used in mining kaolin and coal.
Hydraulic mining developed from ancient Roman techniques that used water to excavate soft underground deposits. Its modern form, using pressurized water jets produced by a nozzle called a "monitor", came about in the 1850s during the California Gold Rush in the United States. Though successful in extracting gold-rich minerals, the widespread use of the process resulted in extensive environmental damage, such as increased flooding and erosion, and sediment blocking waterways and covering farm fields. These problems led to its legal regulation. Hydraulic mining has been used in various forms around the world.
Water was used on a large scale by Roman engineers in the first centuries BC and AD when the Roman Empire was expanding rapidly in Europe. Using a process later known as hushing, the Romans stored a large volume of water in a reservoir immediately above the area to be mined; the water was then quickly released. The resulting wave of water removed overburden and exposed bedrock. Gold veins in the bedrock were then worked using a number of techniques, and water power was used again to remove debris. The remains at Las Médulas and in surrounding areas show badland scenery on a gigantic scale owing to hydraulicking of the rich alluvial gold deposits.
Las Médulas is now a UNESCO World Heritage Site. The site shows the remains of at least seven large Roman aqueduct of up to in length feeding large supplies of water into the site. The gold-mining operations were described in vivid terms by Pliny the Elder in his Natural History published in the first century AD. Pliny was a procurator in Hispania Terraconensis in the 70s AD and witnessed the operations himself. The use of hushing has been confirmed by field survey and archaeology at Dolaucothi in South Wales, the only known Roman gold mine in Great Britain.
Early placer miners in California discovered that the more gravel they could process, the more gold they were likely to find. Instead of working with pans, sluice boxes, long toms, and rockers, miners collaborated to find ways to process larger quantities of gravel more rapidly. Hydraulic mining became the largest-scale, and most devastating, form of placer mining. Water was redirected into an ever-narrowing channel, through a large canvas hose, and out through a giant iron nozzle, called a "monitor". The extremely high pressure stream was used to wash entire hillsides through enormous sluices.
By the early 1860s, while hydraulic mining was at its height, small-scale placer mining had largely exhausted the rich surface placers, and the mining industry turned to hard rock (called quartz mining in California) or hydraulic mining, which required larger organizations and much more capital. By the mid-1880s, it is estimated that 11 million ounces of gold (worth approximately US$7.5 billion at mid-2006 prices) had been recovered by hydraulic mining .
Cities and towns in the Sacramento Valley experienced an increasing number of devastating , while the rising Stream bed made navigation on the rivers increasingly difficult. Perhaps no other city experienced the boon and the bane of gold mining as much as Marysville. Situated at the confluence of the Yuba River and Feather River rivers, Marysville was the final "jumping off" point for miners heading to the northern foothills to seek their fortune. Steamboats from San Francisco, carrying miners and supplies, navigated up the Sacramento River, then the Feather River to Marysville where they would unload their passengers and cargo.
Marysville eventually constructed a complex levee system to protect the city from floods and sediment. Hydraulic mining greatly exacerbated the problem of flooding in Marysville and shoaled the waters of the Feather River so severely that few steamboats could navigate from Sacramento to the Marysville docks. The sediment left by such efforts were reprocessed by Dredging at the Yuba Goldfields, located near Marysville.
The spectacular eroded landscape left at the site of hydraulic mining can be viewed at Malakoff Diggins State Historic Park in Nevada County, California.
The San Francisco Bay became an outlet for polluting byproducts during the Gold Rush. Hydraulic mining left a trail of toxic waste, called "Tailings," that flowed from mine sites in the Sierra Nevada through the Sacramento River and into the San Francisco Bay. The slickens would contain harmful metals such as mercury. During this period, the industrial mining industry released 1.5 billion yards of toxic slickens into the Sacramento River. As the slickens traveled through California's water arteries, it deposited its toxins into local ecosystems and waterways.
Nearby farmland became contaminated, which led to political pushback against the use of hydraulic mining. The slickens flowed through the Sacramento River before depositing itself into the San Francisco Bay. Currently, the San Francisco Bay remains dangerously contaminated with mercury. Estimates suggest that it will be another century before the Bay naturally removes the mercury from its system.
Hydraulic mining on a much smaller scale was recommenced after 1893 when the United States Congress passed the Camminetti Act which allowed licensed mining operations if sediment Retention basin were constructed. This led to a number of operations above sediment catching brush dams and log . Most of the water-delivery hydraulic mining infrastructure had been destroyed by an 1891 flood, so this later stage of mining was carried on at a much smaller scale in California.
Hydraulic mining was used during the Australian gold rushes where it was called hydraulic Sluice. One notable location was at the Oriental Claims near Omeo in Victoria where it was used between the 1850s and early 1900s, with abundant evidence of the damage still being visible today.
Hydraulic mining was used extensively in the Central Otago gold rush that took place in the 1860s in the South Island of New Zealand, where it was also known as sluicing.
Starting in the 1870s, hydraulic mining became a mainstay of alluvial tin mining on the Malay Peninsula.Mark Cleary and Kim Chuan Goh, Environment and Development in the Straits of Malacca, London: Routledge, 2000, p.47.accessed 5 November 2009. Hydraulicking was formerly used in Polk County, Florida to mine phosphate rock.George J. Young, Elements of Mining, 4th ed., New York: McGraw-Hill, 1946, p.436-438.
Hydraulic mining is the principal way that kaolinite clay is mined in Cornwall and Devon, in South-West England.
Egypt used hydraulic mining methods to breach the Bar Lev Line sand wall at the Suez Canal, in Operation Badr (1973) which opened the Yom Kippur War.
The facility processes nearly two million tons of tailings each month at a processing cost of below US$3.00/t (2013). Gold is recovered at a rate of only 0.20 g/t, but the low yield is compensated for by the extremely low cost of processing, with no risky or expensive mining or milling required for recovery.
The resulting slimes are pumped further away from the built-up areas permitting the economic development of land close to commercially valuable areas and previously covered by the tailings. The historic yellow-coloured mine dumps around Johannesburg are now almost a rarity, seen only in older photographs.
Uranium and pyrite (for sulfuric acid production) are also available for recovery from the process stream as co-products under suitable economic conditions.
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