River mining: the modern miners golden opportunity

[RKC]

River mining: the modern miners golden opportunity!

By RKC

Miners have always regarded rivers as important sources of minerals and historically, rivers have yielded great quantities of gold, along with lesser amounts of tin and gemstones. And in recent years it has been proven that with the application of modern mining concepts, techniques and technology, rivers remain a worthwhile source of minerals!



Typical Otago (New Zealand) gold dredged from the rivers.

The most notable examples of rivers, where mining has been successfully carried out in recent years are the Shotover, Kawarau and Slate Rivers, and Lyell Creek, in New Zealand and the Madeira River in Brazil. There are many more river deposits currently being worked in countries such as Papua New Guinea, Indonesia, The Philippines, China and Burma, along with many rivers in North and South America and Africa.



The Slate river.

The modern methods of extracting minerals from rivers can be summarized as, the use of diver-controlled suction dredges (that utilize the eductor/venturi principal) and the New Zealand style excavator-fed floating trommel plants, for mining on a small scale. And for bulk mining the lower reaches of rivers on a large scale, bucket dredges and suction-cutter dredges can be utilized.

Bucket dredging requires capitalization costs of many millions of dollars, lengthy start-up times and an environmental assessment process that can take a great many years. Because of these and other features, it is not surprising that bucket dredges are rare these days.

There are presently a number of extremely large suction-cutter dredges mining gold from rivers in South America and Indonesia.

In respect to river mining in general, the lesson of the 1980s was that small is best. Almost without exception, river deposits are complex and erratic in distribution, which dictates that the preferred method of mining be flexible in operation.

Suction dredges are utilized for sampling and for mining selectively on a small scale. These dredges can also be employed very effectively to work in conjunction with excavator-fed floating trommel plants.

In addition, suction dredges can be the only suitable means of mining when access is not possible for an excavator, or when regulations prohibit the use of heavy earth-moving equipment in environmentally sensitive areas.

All sizes of suction dredges are extremely portable, and virtually any river, anywhere in the world, can be accessed for dredging by using helicopters for transportation.



An 8-inch production dredge being used in a Queensland river.


Suction dredges were first manufactured commercially in California during the 1950s, and since then have been considerably improved in design and performance.

The suction dredges in use prior to the mid 1980s were primarily designed for recreational use, and as a consequence had design compromises that these days would generally be considered totally unacceptable. The style of suction dredges suitable for professional use are identified as Production Dredges! Its important to distinguish between suction dredges used for professional mining and the smaller much less efficient dredges commonly used recreationally, if dredging is to be carried out efficiently and on a viable basis.

Some examples of the innovations introduced during the late 1980s on Production Dredges include; the use of oversize and slightly flexible power jets; the replacement of header boxes with jet-flairs (diffusers) to improve fine gold recovery; the replacement of triple sluice boxes with single boxes; the use of flexible suction hoses, which apart from reducing fatigue for the dredge operator, have the added advantage of being virtually blockage free; much improved recovery systems which permit larger more powerful motors to be used, which then makes it possible to increase suction and dredge size to process greater quantities of gravel; the use of dry suits and wet suit heaters to permit winter dredging; the use of manifold vacuum priming on pumps; electric push button starting on motors; etc, etc. Most of the above improvements are now standard features of a modern Production Dredging operation.

The innovations which make up modern suction dredges evolved from work carried out in North American rivers, and for this underwater miners from all over the world are indebted to the North American miners for their enterprise and high level of professionalism. These innovations were first introduced commercially by the smaller dredge manufacturing companies, and have now been taken up by the industry generally.

The basic concept of a suction dredge is to lift mineral bearing gravel from a river bed, pass it through a recovery system, and to then return the waste gravel back into the river in as short a distance as possible. Each dredge component must form an integral part of the complete unit, with the pumps/s, suction velocity, and recovery plant designed as a whole, to form a complete and efficient unit to suit the deposit, operating conditions and production required to permit viable operation.

The most exciting and potentially revolutionary dredging development is the introduction of 'efficient' 10-inch Production Dredges. In theory it has always been possible to construct 10-inch dredges by just proportionally increasing the overall size of a dredge, along with an increase in the horsepower of the motor/s. However, experience proved that by just increasing the overall size of a dredge, a dramatic reduction in the efficiency of the mineral recovery system resulted. This necessitated the development of radically improved recovery systems. The recovery system in use on the latest design of 10-inch dredges fundamentally consists of first classifying the gravel and then carrying out a controlled secondary recovery of the fines. The system of secondary recovery can vary among dredge manufactures. For example, jigs and a dewatering system can be used, or the fines can be pumped to a separate sluice box with a reduced and controlled flow.

A skilled and experienced dredge operator is absolutely essential if a dredging project is to be carried out on a viable basis. An operator must be continually aware of the 'richness' of the gravel being processed, which allows him to mine selectively. Commercial suction dredging is never simply a matter of vacuuming gravel from a riverbed! An experienced operator skilfully uses his dredge to process only the richest gravel (the pay streak), and while doing so, making sure he leaves untouched the ground he considers of poor value or is barren. By diligently sinking prospecting holes, a 'pay streak' can be located and its outer limits determined quiet accurately. And this skill, like any skill, comes only with experience.



A 7-inch production dredge.

Suction dredge operators refer to the richness of a river in terms of the ounces produced each day; whereas, in contrast, the larger scale miners (using heavy earth-moving equipment) refer to an 'average' weight of gold to each cubic meter of the 'total' alluvium in a river.

River mining, on a scale considerably larger than what is possible with any size suction dredge can be carried out by the use of excavator-fed floating trommel plants. These plants utilize an excavator, which digs alluvial gravel from below water level. The operating principle is to introduce feed into a rotating trommel for primary ore sorting, with the oversize (+15mm) going to tailings. The fine material and water pass through the wall of the trommel, then over a boil box which is designed to catch any larger gold pieces, to the recovery system. The recovery system can vary from the use of a single sluice box on small plants, to the use of jigs, riffle tables, New Zealand style hydraulic riffles, hydro cyclones and spiral bowls in sequence, on the larger plants.



A New Zealand trommel being used on the West Coast of New Zealands South Island.

Because of the impressive quantities of gravel, which an excavator-trommel plant can process, a river, which contains representative gold values of just a few grains to each cubic meter of river wash, can be profitably mined. And if the bedrock is smooth and undulating, nearly all of the gravel contained within a river can commonly be mined.

Excavator-fed floating trommel plants are a method of mining that has revolutionized river mining practice and these plants have a big future in many countries. The initial development work to perfect these plants was carried out in New Zealand during the 1980s and the plants now in use are operating at a high level of efficiency.



A New Zealand trommel operation.

Suction dredges and the New Zealand trommel plants can be used in conjunction to mine a river and their complimentary use is especially applicable in rivers that have hard bedrock. Excavators are unable to extract minerals from hard irregular bedrock. The excavator bucket can do no more than scrape over the high spots such as rock pinnacles and cannot reach down into any cracks and crevices as a suction dredge can.

Therefore the technique most effective for mining a river with hard irregular bedrock, is to first remove the overburden by an excavator for processing through a mobile trommel plant and then mining the exposed bedrock deposit (often the richest of the enriched zones in a river) with a suction dredge. River deposits mined in this way result in a near 100% mineral recovery.

Because of the efficiency of operation possible these days using Production Dredges and excavator-fed trommel plants, either operating separately or in conjunction, large returns are possible with comparatively low capitalization costs.
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Copyright: all rights reserved. This article was first published in the Eureka Echo (The official journal of The Prospectors and Miners Association of Victoria, Inc)



Regards,
Rob (RKC)