Waterjetting 21c - collection in Gilsonite and steep seams

Picking up the debris from a waterjetting operation can be a more expensive part of the process than actually removing the material in the first place. This is particularly true where the material no longer has any inherent value, but is sufficiently toxic that it has to be collected and then properly disposed of. On the other hand, in most aspects of mining, it is the inherent value that is contained in that debris that makes the whole process worthwhile, and in this latter case requires that as much of the material be initially collected as possible. This can be difficult where, for example, gold particles are being produced, since these are heavy and naturally rapidly sink through the water to settle onto the floor where, if this is rough, they can then lie is natural traps that make it hard to re-elevate them and move them to a desired collector.

If one is to continue to move the particles there must be some mechanism that will continue to move the particles to the point that they can be collected. The very condition that makes it difficult to manually mine coal from steeply dipping beds also serves to carry coal and other minerals from those veins, once the material is broken free from the solid.

Consider, for example, the early mining of Gilsonite, which is a naturally occurring hydrocarbon found in thin vertical veins in places such as Utah. It is named after one of its first developers, Samuel H. Gilson.

Figure 1. Pieces of Gilsonite (Metroexpand )

The problem with mining this material is that the dust is very explosive, and remote, safer methods were needed to get men away from the mining process. In 1957 the major American company, American Gilsonite, turned to the use of hydraulic mining. At first the seam was mined using a series of horizontal lifts, with the washed out material and water being collected in drifts that carried it to a pumping station. However, because of the vertical seam, it was easier to used a drill, with lateral jets so that the rotating waterjets mined and flushed the material down into underlying drifts, where it could be collected.

Figure 2. Method for Mining Gilsonite.

By moving the mining tool along the vein the material could be mined out, without miners at the working face, and without the risks of explosion from the conventional mechanical mining methods.

A recent blimp trip over the Bonanza mine shows the results from years of the mining process, as the seam has been removed.

Figure 3. Mined out seam of Gilsonite at Bonanza, Utah. (J.S. and S.W. Aber)

More recently the mine has returned to a mechanical method for mining the material since the customer wanted a dry, rather than a wet product, and, with demand for the material rising, hopefully they will continue to operate safely – the material is now airlifted to the surface. Mining has been restricted to the top 500-ft of the vein.

The hydraulic mining process demonstrated here shows some of the advantages of the use of waterjets in that there are no workers in the mining area, and that the tool is able to remove the mineral without generating the sparks and explosive dust clouds that can be a danger otherwise.

Figure 4. German coal seam side view, showing thickness and angle of slope. (Benedum, W., Harzer, H., and Maurer, H., "The Development and Performance of two Hydromechanical Large Scale workings in the West German Coal Mining Industry," paper J2, Proc. 2nd Int. Symp. Jet Cutting Tech., BHRA.)

Coal mining has some of the same problems, and in Germany the idea was tried (in a slightly modified version) as a way of mining coal. Rather than operate from a drilling rig at the surface, holes were drilled along a very steeply dipping seam. The process was in two parts, evaluating whether it was better to start at the bottom and drill up, or to start at the top and drill down.

Figure 5. Methods tested for bore and mine coal recovery. (Benedum, W., Harzer, H., and Maurer, H., ibid.)

The method where the drill goes down, and then reams back up turns out to be the better approach. The reason for this is that the coal breaks out along natural planes, and these can produce coal pieces that are large enough to block the drilled hole through which all the material must pass, when drilling upwards. This is not a problem when drilling down, since the coal is falling through a much larger space (the mined out volume). This makes it a little more difficult to recover, since it is not confined where it falls into the lower drift. But, by adding a small monitor in the second drift, the broken coal can be lifted back into a slurry that can then be channeled into a flume, and carried away from the working area.

The process was effective, in that it showed that the coal could be mined from one drift to the next, using the method, but it was difficult, because of the varying conditions of the seam over its length, to make the connecting drill holes at an economic speed, and thus to mine the coal at an economic rate.

Figure 6. Looking down to the lower drift from the upper, after mining, with all the coal removed. (Benedum et al ibid)

Note that in the above picture there is little roof collapse into the mined out cavity. This is either beneficial (since it allows all the coal to be removed) or it can be a disadvantage. Other than the safety factor of having a roof hang up until a very large area collapses at one time (where the air blast can do considerable damage), a continual roof collapse as the coal is mined can provide confinement for the water and, hopefully, enough restriction that the water will carry the coal to a collection point.

This was the idea behind the JPL coal mining worm that they proposed some years ago:

Figure 7. The JPL concept for a horizontal jet mining system (Miller, C.G., and Stephens, J.B., Coal Worm: A Remote Coal Extraction Concept, JPL Report 5010-7, December, 22, 1976, Jet Propulsion Laboratory, Pasadena, CA.)

Unfortunately when a coal mine roof collapse occurs the rock breaks into relatively large pieces so that the water can flow away through the pile, without being confined sufficiently to provide the needed motive force to move the coal to the collection point. For that, the system would still need a steeply dipping bed, and I’ll talk more of this and debris collection in future posts.

Longwall Mining - using hydraulics to modernize mines

In writing about longwall mining, I have gone into some detail as to how the technology initially developed, in part because of my own interests, but also because some of its features help answer questions about today’s industry and this will provide a background for those answers in future posts.

The change in longwall came about as I went to college. The manual loading of the coal and setting of roof support that I have described in earlier posts, ended as a result of a single technical change. The miners had loaded coal onto a rubber face conveyor, that was advanced by being split into sections, moved forward, and rebuilt every day. This is incredibly inefficient and the innovation was the change to what has become known as an armored face conveyor. Basically this consists of a series of 5-ft long, hinged elements or pans, connected together in such a way that each can move forward and twist slightly, relative to its neighbors. ( In using pictures of existing equipment let me just insert that they illustrate what I am discussing, but that shouldn't let you think that I believe that one particular brand is better than another.) A single element of a relatively modern version looks like this:

View of a 5-ft long element of an armored face conveyor (Joy )

Horizontal link bars fit between elements in the hollows that can be seen at the ends of the piece allowing flexibility in the joints. The face of the conveyor is the wedge shape, that faces the coal face. Coal is loaded onto the conveyor from the mining machine, and any coal left on the floor then rides up the wedge onto the conveyor, as it is hydraulically pushed forward after the machine has passed. (A point I will discuss later). The coal falls into the top trough of the conveyor.

Pin connection between conveyor pans (Bucyrus )

To move it along the conveyor, there are two chains, which connect to lateral steel bars called flights, which slide in the slots on either side of the top trough.

Top view of conveyor showing flights (Joy)

The chains drag the flights down the trough, pulling the coal that falls onto the conveyor with them, and at the end of the conveyor, the bed is raised, so that the coal is dumped onto the belt conveyor that runs in the entry tunnel to the longwall. (For historic reasons this is generally referred to as the Maingate of the longwall.) The empty chain and flights then return along the face in the underside compartment to the conveyor.

The moving parts of the conveyor are thus now the chain and flights, and they are driven by a motor mounted in the maingate of the longwall.

The arrival of this then new tool allowed a number of different changes to occur on the face. The first and initial requirement was for a way to move the conveyor forward after the coal had been mined out it front of it. It was too big and cumbersome to disassemble and reconnect, and flexible enough that it could be pushed over using a series of hydraulic rams, one attached to each of the pans of the conveyor. But what could provide the resistance against which the rams could push?

The only element in the face behind the conveyor are the roof supports. So these also had to be modernized so that they could work in this way.

Initially the supports had been individual elements, two props and an overlying bar, all made out of wood. I had mentioned last time the arrival of hydraulic props, and steel bars. By combining these into a structure, with the props set on a bearing plate, and the steel bar at the top permanently mounted to their upper section, a simple support element was created. This, as we found out, didn’t have much lateral stability, and so they were combined in sets of two. With a hydraulic ram on the front, this could now push the conveyor forward.

Early hydraulic support (note the ram at the bottom left)

The ram would also make it possible, after the props were lowered, to pull the entire unit forward, and then, after raising the props, reset closer to the face. The plates at the back then stopped the rock that was collapsing into the void left behind the chock where the coal had been been, from rolling into the working face. This became known as a chock.

Over time the props became larger, and the canopy which was meant to extend over the conveyor to provide early roof support didn’t work that well. The idea was that it could be slid out, after the machine had mined the coal, until the chock itself could be moved. The problem was that if there was a heavy load applied at the tip it would bend the cantilever so that it could not slide. Then life became a little more difficult. The answer came from Hungary initially. It was known as a shield support.

Early shield design (dimensions in mm)

The design was extremely flexible and able to mine under conditions such as a badly broken roof that would not have been possible with normal supports, since the top and back (canopy) of the shield provided a virtual complete coverage of the roof and back of the working area, and the shields were installed adjacent to one another.

There have been considerable advances in design, but of an evolutionary nature, since this particular breakthrough – which occurred in the 1970’s. But in combination with the use of stronger and more powerful face conveyors these supports were able to make high-speed longwall possible in a wide range of differing conditions.

It is even possible to mine out a thicker seam by taking the lower coal with a mining machine (of which I will write next time) and then letting the upper coal collapse onto the back of the support, from which it can be recovered using a second conveyor. (And not a lot of energy – Charlie).

Top coal recovery – as in the Hunter Valley of Australia (Bucyrus )

Support used for top coal recovery - its a lot more complex than a wooden prop.



As I mentioned above, I’ll talk about the mining machines next time.

Controlling the roof - more on longwall technology

In describing the earliest types of longwall I mentioned that the evolution came from initially putting different mining sections into a line as a way of simplifying the ventilation and the haulage of supplies to the miner, and coal that he had filled into tubs. This is a layout showing one of those early operations, and the roof is temporarily held up either by pit props, typically around 4 – 6 inches in diameter, and cut to length as needed, or small packs. Because the miner was penalized if there was too much stone in the tub (the entire tub would not be counted) he could stack the stone he found in small packs, with the larger stone on the outside and small stuff shoveled within to provide the roof some support, as needed.

Mining occurs in the open spaces, the small squares are the roof supports and the dark line is where the putter would bring the tubs and supplies or leave with the coal.

The first transition was to change the direction of mining so that it wasn’t perpendicular to the goaf (or collapsed waste) but parallel to it, and then to join all the individual stints into a single face. At this point the supports became more organized and regular plans were developed to ensure that the roof was supported. (These were assembled in a book that the mine manager had to certify, with the conditions for each face specified, and any special support locations laid out.) An early version of such a plan showed the plan and spacing of the props.


The face is shown after it has been undercut, but before it has been fired, and the coal blasted down. In addition the back set of supports would be removed, and the conveyor advanced, before the shot went off. After the back row of supports were removed, it would look more like this, and more typically undercutting and inserting the powder would take pace after the move.


The path of the coal cutter can be seen, and thus use of square sets of timber that were used in bad ground to help support the roof long enough to remove the coal. These replaced the stone piles that had been used earlier. Personally I did not work on faces that used them, except when we had a major face collapse, and we had to mine through it – but that’s another story.

Chock location and construction.

Once the back set of props was removed and the conveyor split, moved forward and remade, then the coal was blasted down and then, as each man loaded out his fifteen yards of coal, the new set of supports was installed.


The reason we could get by with using wood was that the process was relatively slow, and we would mine only one cut a day. This gave time for the roof stresses to redistribute, and the collapsed roof to build up and support the overlying strata.

Section through the face (lower left) showing how the roof breaks, bends and ultimately resupports itself.

As mining began to mechanize, one of the first things that changed was the material of the roof supports. The first step was to change the wooden bars on top of the props for steel ones. Sounds like a good idea, right?

Well imagine that you are kneeling on the floor and you have to erect one of these supports. You put the long bar on your shoulder,pick up the axe and a wedge in one hand, the prop in another, and raise your body until the bar is pressed against the roof. Then you put the prop in the middle, and slide the wedge on top, and tap it tight with the axe. With the bar then stabilized, temporarily, you get a second prop and put it at one end and tighten it into place, and then do the other. Then you remove the central prop. With a wooden bar it was relatively easy in heights from 2.5 ft to 5 ft. With the greater weight of the steel it was harder to move, more critical that you be near the middle when you start, and more vital that the first prop was close to the middle. With a wooden bar there was also a little flexibility in driving in the wedge, absolutely none with the steel bar. Oh, and since we were re-using these (pulling them out of the waste as the last row was collapsed) they pretty soon got bent and so had to be straightened a bit before they could work.

But that was the easy step (in retrospect) because the next thing to replace was the wooden prop. Both the wooden prop and bar were easy to move. But to replace the prop the first idea was to use a friction prop. Here’s how it worked:

The prop was made up of a sliding central square section (above the horizontal wedge in the first picture) that slid within a lower section (where the lifting wedge is inserted).

You lifted the central section until it was about right, then tapped the lifting wedge until the prop was tight against the bar. Now you could move to one side and tighten the horizontal wedge that gripped the sliding section and held it in place. Then you removed the lifting wedge. And when you wanted to remove it, all you had to do was tap the horizontal wedge.

It had the advantage that as the roof stress changed so the prop was intended to yield, with the inner section sliding within the friction band, rather than failing as the wooden prop would if the stress got too high. However, you might want to remember that here we were doing this individually in relatively low coal (at least we didn’t have to saw them to length) with a 6-ft steel bar resting on your shoulder while you try and drive these wedges in. It was a learned art, one of the first things to learn was not to hold the prop above the wedge while you tap it out. The prop drops when released and will hit your hand – usually the thumb – and it has the weight of that steel strap on top of it. Funny how you still remember!

They didn’t last long, for a variety of reasons, but were replaced with the forerunner of today’s roof support, the hydraulic prop. The first ones were relatively simple, and heavy, but much easier to use.


Simply you put the pump handle in the socket, and started pumping. This moves fluid from the upper reservoir to the lower, pushing the central cylinder and the top cap up, and against the prop. To release it you put the handle in the link, and pull, and slowly the prop sinks. Much easier to work with.

But that was only the start, and moving these by hand was still slow, and so the transition to full mechanization began. And before long instead of the simple hydraulic prop, the roof was supported by massive, and powerful hydraulic chocks, but this also required other changes to mining practice. For, as it was said at the time, “We cut the coal by machine, we transport the coal by machine, isn’t it stupid to still load it by hand?”

Schematic view along a face showing a more modern roof support.