Waterjetting 21d - confined coal transport

Collecting the material that a high-pressure waterjet has dislodged from a surface can be carried out in a number of ways, depending on the scale and volumes of material that have to be removed. One of the initial problems that arise depends on the energy of the jets that are striking the target, and the part of that energy that remains in the water and dislodged particles after the jet impact.

If a surface is relatively smooth (think for example of a ship hull, or the deck or sides of a bridge) then when a jet has hit the surface and removed the small amount of material (such as rust or paint) it will likely continue in a relatively straight line forward, since the surface roughness of the target, while disrupting and flattening the jet, has not sufficient angle to radically change much of the flow of the jet.

Which might make it time for a little recap. One of the experiments that I would run with an introductory undergraduate class was to give each student a high-pressure lance, and then have them hold the nozzle just above a target surface. The pressure of the water being fed to the gun was then slowly raised, and as this occurred the jet went from striking the surface and then just flowing along it, when there was no penetration or surface material removal, to being reflected back at the lance holder.

The reason for this is that, as soon as the jet started penetrating into the material (usually a rock for the purpose of the demonstration) then the water is entering a hole where the only exit is back the way that it came in. It is a salutary lesson for the lance holder since all of a sudden the jet is coming straight back (which is why all the personal protective equipment is an important part of the lesson).

This only holds true where the jet is hitting a relatively flat surface in an approximately normal or perpendicular axis of attack. In the more general case the cut along the surface will cause the water and debris to scatter in a more general spread, and it becomes a more difficult job to collect both back together in a way that allows both to be contained and removed from the site (an increasingly important part of the environmental parts of the process).

There are places, such as steeply dipping coal seams, where the geometry of the excavation itself helps to confine this ejecta and direct it, under gravity, to fall into a narrow space where the water and coal particles are brought together so that the coal is suspended in enough water that it can then be carried away from the work zone.

Perhaps the best example of this was the Sparwood mine in British Columbia in Canada, where the mine was extracting coal from a seam that was roughly 40 ft thick, and which dipped at around a forty degree angle.

Figure 1. Section showing the Sparwood mining plan

Drifts were first run at a slight angle (this started at six degrees, but after lining the flume with Teflon plates the mine was able to reduce this to just over four degrees) to the strike of the seam. This was a sufficient angle that, when all the coal was caught in the flume it would be carried down without settling by the spent water from the mining process, which was also trapped in the underlying drift, and held by a barrier across that drift. The shallower the angle then the more coal could be recovered above the main haulage ways at the back of the working area.

The mining tunnels (drifts) were first driven to the back of the section, using a small road heading machine to extract the coal, while installing a flume along the side of the drift so that the coal could be immediately transported away as it was mined. Full support to the tunnel was also installed using arch girders, with bracing wooden slats between the girders. Once the drift had reached the end of the seam, then a hydraulic monitor was placed in the uppermost drift, and the arch girders and wooden planks removed from the final fifty feet of the tunnel, with the monitor placed under the last few tunnel supports of the remaining tunnel section.

Figure 2. Layour of the monitor within the access drift.

By using a jet of just over an inch in diameter. the jet was able to reach the back of the section of coal that had been exposed when the supports were removed (zone 5 in figure 2) a distance of over 120 ft. the monitor was moved by two sets of hydraulic rams, but if you note where the operator is standing at the back of the machine, this is some 40-ft from the opening and the mining operation itself is not visible.

Figure 3. A monitor in operation at Sparwood. Note the short length of the barrel, which would still produce a high-quality jet, since flow straighteners were used in the barrel, placed directly behind the nozzle.

The operator uses the rams to move the nozzle in an oscillatory path, and listens to the sounds of the jet as it strikes the coal. The sound is quite distinctly different when the jet is hitting coal, as opposed to striking roof rock or shooting into the open space of the drift updip. (I was told this, not having that experience, though I have found similar changes in sound useful in other applications that I will discuss from time to time). It takes, apparently, a couple of days for an operator to be able to consistently detect and use the sound differences to be able to effectively mine with the monitor.

Figure 4. Operator at the Sparwood mine, standing at the back of the machine, and beside the flume.

The way in which the coal broke under the jet attack was only controlled by the operator to a limited extent, so that there can be a significant volume of large coal surviving into the lower entry for collection, and flume transport needed a smaller size distribution. For this reason the coal company installed a coal breaker at the entry to the flume so that the water carried the coal lumps through the breaker, and only then did they enter the flume (Figure 4).

At the time that I visited the site the slurry was higher than shown in the above figure, with coal overlying parts of the back of the breaker. To make it easier to operate the breaker, while keeping the operator safe behind the roof supports, a second small monitor was set by the operator which could be used to clear off the machine from time to time.

The machine was operated by two individuals and over the course of ten years averaged a production of over 3,000 tons a shift. It was also for many years, the safest mine in Canada. To put that production in perspective, in those years an average section in the underground mines in Illinois, running a continuous mining machine, might average about 700 tons a shift, and would need about 14 men to achieve that target. (Production rates have since risen considerably as automation and remote control have reduced the number of folk needed, while higher-powered machines now produce greater coal volumes faster.)

Yet the range of seams where this type of mining can succeed is limited, particularly in the United States, and in coal seams that do not dip as steeply it becomes more difficult to control the particle and water spread as it leaves the impact region.

I’ll talk about a specific way that one can, on occasion, change that, in the next post.

Waterjetting 2d - Adding cracks to Nature

In the last few weeks I have focused on demonstrating, with examples, that water effectively removes material by penetrating into natural cracks in the material and causing them to grow. But what happens when there are not enough cracks to remove material at an economic rate? The modern approach has been to raise the pressure of the water so that smaller cracks grow faster, thus providing the production rates needed, but that option wasn’t available in the past.

I mentioned last time that miners in the Caucasus Mountains of what is now Georgia used the power of mountain streams to erode gold deposits over 3,000 years ago. Perhaps learning from that, when the Romans came to Las Médulas in Spain, some 2,000 years ago, they though of water again as a way of mining the gold-bearing sandstone of the local hills. And though they had to modify the initial idea, the result became the most important gold mine in the Roman Empire. It is now a World Heritage Site.

Figure 1. Location of Las Médulas in Spain. (Google Earth)

The sandstone was more resistant than soil, and so the Romans came up with two ideas to improve the rate at which the gold ore could be removed. The first idea was to run galleries into the sides of the hills, creating large chambers underground, with support for the roof from wooden supports that were left in place.

Figure 2. Tunnel driven into the bottom of the hill at Las Médulas.

Figure 3. Underground room at Las Médulas.

At the same time that the mining preparations were going on local streams were being diverted and dammed so that a large volume of water was held in reservoirs and then carried by manmade channels to a point over the mining chambers. With the water ready, the timbers were set on fire, which initially weakened the overlying rock so that it began to fail, falling into the opening, and as the support burned away more rock fell into the opening until the cavity worked its way up to the surface. At this point the reservoir gate was opened and water flooded down the channel to fall into the cavity. As the water fell it further broke the rock into grain-sized pieces, and carried these down and out through the original opening in the hillside.

Figure 4. A Collapsed cavity, not the two figures at the arrows to get a sense of scale.

The water and debris flow was directed into flumes, in much the same way as modern miners in Alaska practice today, except that where carpet is used to catch the gold particles in Alaska, in Spain the Romans used plant stems (silex) to catch the gold. After drying the plant could be burned, easing to recovery of the gold. (In more modern times Spanish miners have lined the flumes with oxen hides.)

Figure 5. Artist sketch of the troughs used to capture the gold particles at the Spanish mines.

The use of heat to weaken rock before using water pressure for cutting has been tried with a couple of interesting wrinkles both by researchers at Rolla, and at the then U.S. Bureau of Mines and in Colorado, among others. But those more modern trials will be described later in the series. Using water streams to erode surface outcrops of mineral survived as “hushing” in the North of England and elsewhere until fairly recently.

Move forward some 1800 years or so from Roman Spain, and at the turn of the 19th Century miners in both Russia and New Zealand had a problem in mining coal. In both countries there were good quality coal seams, but they sloped at a steep angle that made it difficult to move men around without their slipping and falling. It was also difficult to support the roof, which was achieved at the time by sawing wooden props to length and wedging them between the roof and floor. Both nations had the idea of modifying the Roman idea of using water to remove the mined coal, but coal was thought to be somewhat stronger and more resistant than the Spanish sandstone.

In the New Zealand case the mountainous countryside makes it expensive to drive roads and as early as 1891 wooden flumes were being used to carry coal to the consumer. However it was then realized that the water could be used to also remove the mined coal, particularly that which was left in regions of the mine where it was not safe for men to go. The coal was therefore initially blasted, and then the flow from the nearby streams was directed at the debris pile. The volume of water, and the slope of the mine combined to remove all the mined coal, often overnight, so that a new area could be worked the following day. It was not until 1947 that pumps began to be used to drive the water at greater pressures. At this point, with the higher pressures that pumping brought, it was no longer necessary to pre-crack and break the coal with explosives.

While the New Zealand coal seams outcropped at the surface in very hilly ground, the situation was somewhat different in the Donets coal seams in the Soviet Union, where the seams were thinner, and production was barely economic. The seams in these mines were much deeper than in New Zealand, and so jet pressure could be provided from the drop in height from the mine surface to the location of the large nozzle or monitor that was used to aim the water flow at the coal. As with the New Zealand experience the Soviet miners (at the Tyrganskie-Uklony mine) initially blasted the coal with explosives to weaken it with a high density of cracks, before applying the water. However the miners found that not only did the water double production (to 600 tons/shift) the streams were powerful enough that it wasn’t necessary to pre-blast the coal. The nozzle diameters of the time were up to 2-inches in diameter, and could throw a jet up to 60 ft.

Figure 6. Early Soviet underground coal miner

It was from these small beginnings that hydraulic mining began, it was, in its time the most productive method of mining gold in California, and was used for many years around the world for mining coal, and other minerals. But that again is a subject for more detailed discussion at a later time.

The combination of explosives and water power remains in use in harder rocks, particularly in South Africa in the gold mines. Here again the seams of gold are very narrow and can slope or dip at a steep grade, the working area is thus kept very cramped and difficult to work. By blasting the ore with explosive, it can again be moved with water pressure, although there is an additional advantage to water here that I will further explain when I write about cleaning rust from plates.

Gold, as is shown by the way it can be collected in flumes, is very heavy, and part of the problem in the South African mines is that small pieces can get trapped in small pockets on the floor of the seam. The higher pressure water flows can flush out these pockets driving the gold particles down to a common collection point. In that particular the practices haven’t changed that much in three thousand years.