Waterjetting 21b - collecting debris, or mined product

You don’t have to operate a water jet lance for any time before realizing that the rebounding water, and the debris from the operation can’t be ignored. On the other hand, after they fall to the floor they rapidly lose their energy and without some additional input will lie in the most inconvenient places until moved. Protection from the rebound is a critical part of the protective clothing and equipment that the operator wears (including side shields for the eye protection) but the collection and removal of the spent material is also critical to good operations, and in uses such as mining it is needed to carry the material to a point where it can be processed.

While this was fairly obvious to those who used water in earlier times to wash out soil and minerals and carry them down to flumes and separation devices, it was not always immediately obvious to those who experimented with the use of larger jets for underground mining. There are two critical velocities involved, once the particles are removed from the surface, these are the settling velocity and the suspension velocities of the particles. Essentially these are the speeds at which the transporting water is travelling that prevents, or encourages, the particles to settle out. This is an important part of drilling horizontal holes, where the drill cuttings can settle in the long hole hehind the drill, and can both stop the drill from moving forwards and also trap it when it tries to retract (personal experience).

Figure 1. Settling velocities (after VCCS)

Because of the wide range of particles that are mined by a jet, it is sometimes suggested that flow velocities in flumes remain above 40 ft/sec. It becomes a lot more difficult to get particles back into suspension after they have settled out.

Because water and debris are dispersed around the impact point after jet impact and because the amount of water needed to keep the particles suspended and moving has to be high, this often means that the geometry of the excavation has to be tailored to capture and confine the water in a narrow space with the mined product.

The National Coal Board carried out early experiments in coal mining in Wales at Trelewis Drift. (Jenkins R.W. “hydraulic Mining – The NCB Installation at Trelewis Drift” MSc Thesis, University of Wales, 1961).

Figure 2. Early NCB remotely operated monitor (after Jenkins ibid).

The 5-ft 6-inch thick seam dipped at roughly 6 degrees, and the monitor was used initially to drive the drift, and then to slab off coal from the pillars on either side, and then to mine along the face of a pillar, driving coal from one drift to the next adjacent, as the drift was retreated.

Part of the problem that arose was that the coal broke off in pieces that were up to 2-ft thick, and once these fell to the floor they became more difficult to recover, particularly where they were scattered (and this was the reason that the monitor was being operated remotely since the debris was reaching the miners). (Jet pressure was around 1,000 psi). The mine was naturally very wet (over the waders of some miners) and production did not exceed 45 tons a shift, even though the costs at the time were around 50-pence per ton in 1963. The conclusion was that the slope of the seam was not adequate to help enough with coal transport and the experiments were terminated.

This is in contrast to work elsewhere (the British trials had been commissioned after Russian hydraulic mining trials had been reported as successful). But the Russian work was carried out in the Donets coal basin, where the seams are much steeper, and more difficult to mine conventionally.

V.S. Muchnick wrote his dissertation on hydraulic coal mining in 1935, describing these early trials. While it was difficult to manually work in the steep seams, setting up a simple monitor that would wash away the coal, which would then fall under gravity to the drift tunnel beneath the mining operation was much more successful.

Figure 3. Early Russian Monitor (RGM-1)

Because of the Second World War hydraulic mining did not get its start until 1952 at the Tyrganskii-Uklony mine in the Kuznetsk Basin. Production in the hydraulic sections was anticipated to be 500 tons.shift, but by the end of the first year it had already exceeded 600 tons, more than twice conventional mining production. In the early operations the coal was weakened by blasting, but by 1957 jet pressures were raised sufficiently that this was no longer needed.

Figure 4. Showing the major Russian coal basins (Gazprom )

By 1979 there were nine major hydraulic mines in the Soviet Union with an annual production of over 8.9 million tonnes. As the mining operations expanded, so the monitors were increasingly operated remotely using hydraulic cylinders to direct the jets at the coal. At pressures of 1,500 psi the monitors could mine over 50 tonnes an hour, using around 650 gpm of water. (This was later doubled).

In almost all these operations the mine is worked in retreat, first driving the drifts to the back of the section, and then mining back, allowing the coal either to fall into the drift, where the water volume is sufficient to carry it into the flume, or pushing it down to the underlying drift, where it can similarly be collected.

Figure 5. GMDTs-3M monitor used in the Soviet Union

The mining pattern changed with the thickness of the coal, and with the steepness of the slopes at which the seams ran, although the access drifts were run more up dip, while the mining drifts were at a shallower angle closer to horizontal, so as to make working conditions easier, and to more effectively remove most of the coal from the section. In this way the coal collapsed under gravity down to the drift, where the water would float it into the flume. ( A barrier across the drift would confine the water and coal and act as a feed mechanism).

Figure 6. Method of mining in Soviet hydraulic mines in gently dipping seams of average thickness.

I will discuss the expansion of the technology around the world in later posts.

Waterjettting 19b - California gold mining

While mankind has directed the flow of water against earth and rock faces for millennia, as a way of eroding and removing material, it was not until the days of the Gold Rush in California around 1850 that the idea of confining the water into a hose, and through a nozzle crystallized.

Gold was originally found in the gravel in 1848, when James Marshall was helping John Sutter build a sawmill on the banks of the South Fork of the American River, in what is now Coloma, CA.

Figure 1. Sutter’s Mill on the American River (Replica by California Parks ).

The news that gold had been found in the tailrace led into what has been referred to as the greatest mass movement of people in the Western Hemisphere, as people flocked to California over the next few years, during the period of the California Gold Rush.

Figure 2. Location of Coloma, CA (red circle) relative to Sacramento and San Francisco – Lake Tahoe is by the 395 sign in the upper right. (Google Earth)

As the prospectors panned the gold from the stream beds, so they moved north east along the valleys and rivers, seeking the sources of the gold particles that millennia had washed down from the Sierra Nevada. One such source was found at American Hill, just north east of Grass Valley. Here the gold was found in beds of a weak sandstone, lying relatively close to the surface.

Figure 3. The American Hill Diggings, with plaque. The original height of the hill can be seen in the background.

The gold settled to the bottom of the sandstone, and so the miners would tunnel into the side of the hill, seeking to find the richest layer. Unfortunately as you dig out the bottom of a hillside, the overlying rock has a habit of falling down, with mildly fatal results to those caught in its path. This made mining somewhat dangerous, given the soft nature of the rock as Edward E. Matthison found when he was nearly buried when he was working the property. So with partners, he decided that a more remote method of digging out the gold was needed. So, with the help of a local blacksmith named Miller, he fashioned a nozzle on the end of a canvas hose he ran from a water reservoir at the top of the cliff (initially a nail keg) and used the resulting stream to wash the ore (and overlying rock) into a channel that was later turned into a flume, with a series of strips to catch the gold.

Figure 4. Early hydraulic mining

The method had many advantages since, in the process of washing the rock from the solid it was broken down into individual particles. This separated the gold, sand and clay particles, so that while the gold particles would be trapped in the flume, the lighter sand and clay particles would be carried further downstream with the water. By 1853 they were paying a water bill of $153 a week (with water at $0.75 per miners inch this meant they were using 2,000 gal/min) but making the four partners a profit of $50 a day. Larger and larger monitors (the name given to the nozzle and pivoting assembly) were built, throwing water at greater distances, and mining at much faster rates.

Figure 5. Monitors at work at the North Bloomfield mine.

This, in turn, required increasing amounts of water, and this was carried in flumes down through the Sierra Nevada, with agreements being made between companies for distribution, collection and the passing on of water. The nozzle diameters of some of the larger monitors grew to more than 8 inches, and they were capable of mining tens of feet from the operator.

Figure 6. Later design of monitor. The wooden beam usually had a box holding rock on the other end in order to balance the weight of the nozzle section.

The nozzles were made longer, as they were made larger, in order to get the jet to throw to greater distances, but this made steering and control of the jets more difficult. The gooseneck swivel was invented in 1855 to help swivel the nozzle, and a monitor operator noted that when he stuck his shovel into the stream of water it deflected the nozzle. This was Dave Stokes at the Malakoff mine and led to the invention by his Supervisor, Henry Perkins, of the rotating system for sprinklers that is still used to this day.

Figure 7. Modern rotating sprinkler showing the deflection plate. (Aliexpress )

The largest mine in the region was the Malakoff, and in the region around it there were some 425 companies operating and, between 1871 and 1880 they produced $121 worth of gold (at the price of the day).

But there was costs to this operation outside of just the mining ones. For while the gold was captured in the flumes, the sand, and more particularly the clay, was carried in the water until it became less turbulent. And that was when it reached the Yuma, American and Beam rivers flowing out of the Sierra Nevada and down towards Sacramento. As the water slowed, so the clay precipitated out, and the river beds filled with sediment. Thus, when the rains came, the water overflowed its banks, flooding the neighboring fields.

Foregoing the fact that it was the mining that had brought the farmers and many others to the region, the floods were not acceptable, and following the floods of 1880 there was an increasing effort to contain the mining sediments. This led to the court ruling by Judge Sawyer in 1886 restricting the practice of hydraulic mining, and the technology fell into abeyance. It was restarted at the time of both World Wars, but in recent times there was only one small mine that had been “grandfathered” still in production. Its role in developing California is not greatly recognized at present, and the remaining legacy is more seen in the vertical bluffs and large flat areas of mined sand that are left north of Grass Valley, together with the old wooden water flumes that still thread their way around the edges of the valleys.

Figure 8. View of the Malakoff Diggings

I’ll talk more about the spread of the technology next time.

Waterjetting 2c - using Nature's crack system

In this section (part 2) of the series on Waterjetting, the focus is on the way in which high-pressure waterjets grow cracks in their target. As John Field showed, even the presence of microscopic cracks on a glass surface are enough to initiate the larger cracks that lead to failure. In many cases, however, the most useful growth can be achieved if the cracks only extend to the point that they remove a desired amount of material. This becomes important where there are weaknesses and flaws in the material – such as the layers between plies of wood, or even Kevlar - which should not be grown as the jet cuts down through the material. And in a later article this topic will be a part of a discussion as exactly what happens as a jet drills a hole into a target. But, for today, I would like to talk about crack growths in rock and soil, both because it is one of the oldest ways in which water can penetrate into material, and also because it holds the potential to be one of the newest areas into which waterjetting is growing, and will likely further advance into a more significant business.

And to begin consider that, as water penetrates into the cracks in a rock, and grows those cracks slowly, under natural forces, rocks with minerals in them, will see those mineral particles separately broken out. The classic example of this is with gold. One of the ways in which the Forty-Niners found the gold in California was by panning for the gold particles in the rivers, and tracking the gold deposits back up-stream until they reached the original gold deposits of the Sierra Mountains. Not that this was the first time that water transport had helped in gold mining. One of my favorite stories to begin classes is to remind them of Jason and the Argonauts.

Figure 1. Movie poster for the 1963 film version of Jason and the Argonauts (iMDb )

It is a theme that has been made into a movie several times, (see, for example, here) and tells the story of how the Greek Prince Jason and a band of companions go in search of the Golden Fleece, and the adventures that he has along the way. Despite the mythical creatures the story is thought to be likely based on some measure of truth, with the voyage taking place some time before 1300 B.C. But our focus is on the fleece, rather than the voyagers.

Figure 2. Suggested path that Jason followed to get to the River Rhion in Georgia.(Google Earth)

Within the Caususus mountains of Georgia lies the modern town of Mestia, which was thought in Roman times, to be the site of Colchis, where Jason found the Golden Fleece. The reality is not quite as dramatic as the legend since, as the Roman historian Strabo noted

“It is said that in the country of Colchis, gold is carried down by mountain torrents, and that the barbarians obtain it by means of perforated troughs and fleecy skins, and that this is the origin of the myth of the Golden Fleece”

The torrents of water in the Svaneti valley outside Mestia, (Nika Shmeleva Google Earth at 43deg02’29.74”N, 42deg42’25.13E)

It is thought that the miners of the time directed the streams so that they flowed over the veins of gold and eroded out the particles so that the gold was carried down to the valley. Here it was fed through the troughs that Strabo described, and the heavy gold particles were captured as they tangled in the wool of the fleece. To recover the gold the miners would then hang the fleeces in trees, so that they would dry, and the gold could be shaken loose. Unfortunately as the fleeces hung in the trees they provided a tempting target for Greek thieves. (In a later version that I will write about in the next post the sheep fleece was replaced with brush that could be dried and burned to release the gold).

Water was thus, in one of the earliest “automated” mining processes, used to both dislodge and then carry the valuable mineral from the mining site The overall power of water to move soil has been used to wash away material for over a hundred years. In the 1973 War between Egypt and Israel the Egyptian Army gained a significant advantage in the early hours of the war by using waterjet monitors to wash away the defensive barrier along the edges of the Suez Canal, rather than using conventional mechanical excavators.

To deal with the massive earthen ramparts, the Egyptians used water cannons fashioned from hoses attached to dredging pumps in the canal. Other methods involving explosives, artillery, and bulldozers were too costly in time and required nearly ideal working conditions. For example, sixty men, 600 pounds of explosives, and one bulldozer required five to six hours, uninterrupted by Israeli fire, to clear 1,500 cubic meters of sand.

The quoted Sunday Times report of the time suggested that the Israeli Army had anticipated that it would take 24-hours to remove the barriers giving time for their Army to mobilize and arrive. However, using a set of five pumps per breech site the Egyptian Army was able to make an opening in as short as a 2-hour time, with the mobilized water cannon opening 81 breeches, and removing 106 million cubic feet of material in that first day of the war. They were thus able to initially advance into the Sinai with relatively little resistance.

The pressure of the water does not have to be high to disaggregate the soil, but large volumes were needed in that application both to break the soil loose and to move it out of the way. Moving the debris out of the way is an important part of the operation, and while, in the above case it could be just pushed to one side, in many more localized jobs, particularly in cities, that is not an answer. However if the soil can be collected with the water, then the fluid can help to move the soil down a pipe away from the working area. And, more importantly, if the soil can be captured as it is being broken loose, then both can be collected before the water has had a chance to penetrate into the soil around the hole, and so the walls of the hole will not get wet, and will remain stable and not fall in.

One way that we have achieved this is to rotate a pair of waterjets relatively rapidly (depending on the material the jet pressure can range from 2,000 psi to 10,000 psi) so that the surface layer is removed, and to immediately take this away by combining the jet action with a vacuum for removal. (In the initial trials we used a Shop Vac to remove both water and debris). This combination has become known as hydro-excavation, and will be the topic of a couple of posts in the future.

Similarly the use of high pressure to break an ore down into its different parts, so that the valuable mineral can be separated from the host rock at the mining machine, is become a new way to reduce the costs of transporting and processing the ore, and make mining more efficient. As yet this latter is still more of a laboratory development, though it will develop for greater use in the future, and there will be additional posts on this too in the future. But, in both cases, the use of waterjets to effectively rely on extending pre-existing cracks makes the systems work. In the next post I’ll write about a couple of other ways of getting enough cracks into the rock as ways of making it easier to separate and remove valuable materials from underground.