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Voyage to the bottom of the sea

Ocean mining an overview

The 1960’s movie and TV show implied that getting to the bottom of the sea was hard, but very doable, we are still learning how.  In fact, while we have been trying for a long time (2500+ years actually), we still know little about what is there.

The oceans cover almost 75% of the earth’s surface, yet we probably have explored less than 10% of the oceans floor.  In fact we probably know more about the minerals resources of the moon then of the deep ocean floor.  NASA’s Magellan spacecraft mapped 98% of the surface of Venus to a resolution of around 100 meters. 100% of Mars has also been mapped to that resolution and 60% has been mapped at around 20 meters resolution.  The moon’s surface has also been mapped to 7 meters resolution.  While the resolution of deep seafloor mapping is at 5,000 meters.

Mining from the sea floor has been documented for a long time. The earliest records show dredging operations as far back as 500 B.C. in Tyre, Lebanon ¹. Other indications show harbor dredging began during the Bronze Age along the Nile, Euphrates, Tigris and Indus rivers ².  While much of this was for clearing harbors, the recovery of sand and gravel, and some metals is known.  In the past much of this has been in the shallow near shore (less than 50 m water depth), but the industry is evolving and mining in deeper water looks set to proceed,

Still seafloor mining is a relatively small industry since only a small part of the known (or projected) mineral deposits are currently being recovered.  Mining on land (including the few operations that have extended under the ocean) is a major industry. Increasing demand will likely push sea floor mining even more.³

So what methods have, are, and projected to be used.

Much of the earliest dredging was around ports and harbors.  While much of this was for maintain harbor depth, some was used for construction. 

The above figure shows early dredging equipment used in European river and harbor areas. The vessel immediately adjacent to the ‘pull boat’ is equipped with a removable bottom, to facilitate the dumping of dredged material.  The nature of the old dredging scours at Marseilles and Naples indicate that similar methods were employed much earlier (Roman and pre-Roman). The technique had evolved very little since this time.

Currently dredges fall into two kinds, but with many sizes of each, suction and clam shell.   Suction dredges are currently the predominate method for dredging. 

Clam shell operations (and sometimes draglines) have been used for shallow deposit of 20 meters (65 feet) or less.  But even these are being replaced by suction types.

 

They still see use in harbor dredging,

In the past (up to about 40 years ago), a third kind ladder or bucket chain was predominate. They were used in depths up to 60 meters (200 feet).  Both off shore fluvial deposits and on and near-shore placers have been mined this way.    

 

Stacker/Bucket – Typical Gold/Placer

More recently suction dredges have become the main way to dredge. This is due to the relative simplicity of the operation (pump and hose/pipe) as compared to bucket/ladder dredge.  And the ability to be fairly continuous as compared to a clam shell, especially at greater depths.  But that does not mean suction dredges do not have problems.

If the suction is by a centrifugal pump at the surface, the maximum suction lift is about 4.6 meters (15 feet).  With a positive displacement pump the maximum is about 6.7 meters (22 feet).  This can be solved by placing the pump at or near the pipe inlet at the sea floor,  This does increase complexity by requiring a power source to the pump and design issues due to the water pressure.

An alternative method is by using air lift suction, where compressed air is inserted at or near the inlet causing a flow to then lift the material.  There is a practical limit of 215 meters (700 feet) without special design considerations. 

For harder deposits or more consolidated material, suction dredges may have a mechanical cutter head at the suction inlet.

Another point on sea floor mining, that is often overlooked, is the production rate.  While many land based operations achieve production rates of several thousand tonnes per hour (some in the 10’s of thousand), sea floor operations are generally under a thousand, and sometimes around a 100 tonnes per hour maximum. For high value minerals (diamonds, gold, and such) this may not be problem, but does require high grade deposits, especially as deeper depths are mined.

Basic suction dredging is fairly simple, The dredged material is generally placed into an onboard hopper and excess water and tailings are discharged back into the environment. But there are environmental impacts.

Environmental impacts include disturbing the marine sea floor environment, and disturbing marine life with the tailings plume. The most immediate impacts relate to sediment removal resulting in loss of sea life communities. The removal of the sediment may also affect recovery rates of impacted communities. Studies have shown that dredging can result in a 30–70 per cent reduction in species variety, a 40–95 per cent reduction in the number of individuals, and a similar reduction in biomass.

In addition to removal, sediment disturbance and tailings discharge can expose marine organisms to increased turbidity and elevated suspended sediment concentrations. This can reduce light availability, which can impact photosynthetic organisms like phytoplankton. Tides and currents can spread turbidity plumes and sediment beyond the mine area. This can be accompanied by changes in water chemistry and contamination (such as algal spores, and from formerly buried substances).

1.   Geoarchaeological evidence for dredging in Tyre's ancient harbour, Levant, Published online by Cambridge University Press:  20 January 2017, Nick Marriner and Christophe Morhange

 

2.   Mind the (stratigraphic) gap: Roman dredging in ancient Mediterranean harbours, Bolletino Di Archeologia On Line, International Congress of Classical Archelogy 2008 Rome, Christophe Morhange, Nick Marriner

 

3.   Chapter 23 - Offshore Mining Industries from Assessment of Other Human Activities and the Marine Environment, Published online by Cambridge University Press:  18 May 2017, The First Global Integrated Marine Assessment World Ocean Assessment I , pp. 363 - 378

 

o   40+ years’ experience in the mining industry with strong mineral processing experience in precious metals, copper, industrial minerals, coal, and phosphate

o   Operational experience in precious metals, coal, and phosphate plus in petrochemicals.

o   Extensive experience performing studies and determining feasibility in the US and international (United States, Canada, Mexico, Ecuador, Columbia, Venezuela, Chile, China, India, Indonesia, and Greece).

o    E-mail:  info@smartdogmining.com