Waterjetting 37e - Using Cavitation to disintegrate rock

In most mines the main objective is to recover as much of the valuable minerals contained within the host (or gangue rock) while minimizing cost. When miners have to go underground and haul the ore to the surface before the minerals are recovered then there is a considerable expense both in hauling all the rock, including the large otherwise valueless host rock, to the surface, and then crushing it to a small size so as to liberate and separate the valuable components.

The work at MS&T, for a number of years, has looked at ways in which rock can be disintegrated, as it is mined, so that the different components are separated as they are freed from the vein. While this work has progressed significantly since it started, this video (of poor quality for which I apologize, but it was what was available at the time) describes where we started the work.

Figure 1. Tom Fort explains the work on cavitation disintegration of rock

  The work was carried on in a number of ways after that, some of which has been described in an earlier post.

Perhaps most relevant to the video at the top of the piece, we were able to develop a more continuous mining process where the material would be mined from the solid in the mine, rather with small hand samples in the lab. While the technology could be easily developed from existing machines now used for hydro-demolition, a more telling picture is to show, by running the product from a test on a sample of dolomite hosting a vein of galena, where the product was run over a Wilfey table.

Figure 2. Mined sample run on a Wilfey table.

The result shows that clear fragmentation of the galena particles and their liberation so that they form a separate (silver) stream on the table from the darker dolomite particles that lie closer to the riffles. It is not quite as easy to see the larger particles of galena which were also separated, but would be more easily recovered perhaps with a screen, since they are not quite as easily streamed from the dolomite.

Waterjettting 30c - why cavitation isn't being used in machining metal.

Most applications of high-pressure waterjets need the jets to make relatively precise cuts into a target surface. When examining different ways of improving jet cutting performance, the narrowness of the cut which is achieved is therefor often a critical factor in deciding how to make the cut.

In this short segment of the series I have been discussing some of the findings that Dr. El-Saie made in his Doctoral Dissertation, and at the end of the last post had shown that he had found that, under the right conditions, a cavitating jet would remove more material from a surface than would an abrasive-laden waterjet of equal power within the same time frame, and at the same operating jet pressure.

This work was carried out prior to the submission of his Dissertation in 1977, and – because most of the work on refined nozzle design had still to be done – the nozzle designs that were used in the study were considerably cruder than those that have been developed, by a number of companies, in later work. For this reason it is not realistically possible to compare the results achieved in the early studies with the current state-of-the-art, particularly since, while there has been a great deal of work on improving the design of AWJ nozzles there has been almost none focused on improving the cavitation destruction of a waterjet.

The important distinction to make is that while there has been considerable work on reducing the damage that a cavitating flow can achieve when it impacts on a surface, there is almost none that has been aimed at making that damage worse. Part of the problem that this lack of work has left us with is that, in most cavitating systems, part of the cavitating cloud will collapse within the nozzle assembly. While this may only be a small fraction of the total, within a relatively short time (and this has been measured in fractions of a minute in an intense erosion design) the nozzle is destroyed. It is therefore unlikely that the most efficient nozzle design for inducing cavitation erosion has yet been developed.

Part of the reason that it has not has to do with the requirement that is listed at the top of the page. Where abrasive particles are mixed within a very narrow jet of high-pressure water, the abrasive cutting is confined, so that the slots can be controlled to a high degree. We have shown, for example, that it is possible to make cuts through titanium within a tolerance of 0.001 inches of the design requirement, and with a smooth surface over the full surface of the cut. (This requires, in thicker materials, that the nozzle be slightly tilted so that the jet taper over the depth of the cut is compensated for over the desired edge cut).

Unfortunately this precision in cutting cannot be achieved (at least with the current levels of understanding of the process and controls) with a cavitating jet system. In part this is because of the omni-directional nature of the collapse of the cavitation bubbles over a surface. An abrasive particle has the great majority of its velocity aligned with the axis of the cutting jet (though this might slightly deviate if the particle cuts into the surface more than once over the depth of the cut). Cavitation attack can occur in a more omnidirectional way.

To illustrate the point consider an experiment where we fired a waterjet along the edge of a block of dolomite, in such a way that the jet did not contact the rock, were the test to be carried out in air. The jet was carried out underwater, with back pressure of the surrounding water adjusted to intensify cavitation along the edges between the waterjet and the surrounding water. The collapse of those bubbles against the side of the rock eroded the cavities shown.

Figure 1. Samples of dolomite attacked by a cavitating jet. The jet is, in both cases aimed parallel to the cut face (along the line of the red arrow) and just off the block surface.

The samples shown in Figure 1 were exposed to the jet for a minute, with the samples held under water in a cell where the back pressure (BP) could be adjusted.

At a pressure of 6,000 psi, with a back pressure of 60 psi and a nozzle diameter of 0.02 inches the damage to the rock is small. Although it should be noted that there is a hole that eats into the rock perpendicular to the direction of jet flow.

This is more immediately obvious with the sample shown on the right, which was cut with a 7,000 psi jet, against a back pressure of 35 psi, and with a jet diameter of 0.030 inches.

It is also worth noting that the hole generated is not consistent in diameter as the hole deepens. The penetration of the jet into the wall, perpendicular to the main jet flow, is caused by the individual collapse of cavitation bubbles against the surface of the rock. And this is not a consistent phenomenon along the jet length, but for varying conditions it will occur at different distances from the nozzle. In this case the hole widens and deepens about half-an-inch below the rock top surface, eating further into the rock relatively consistently for the following few inches.

In a separate experiment the resulting hole can be seen to vary in depth over the length of the cut into the rock.

Figure 2. Hole cut into dolomite by a cavitating jet. Note that softer layers of the rock are excavated more deeply into the hole wall by the collapsing bubbles. The hole is roughly six inches deep.

The cut is also much broader than the originating jet, as can be seen from the cavity eaten into the rock surface perpendicular to the jet at the bottom of the sample. The cut is much more ragged than the precise line that would be cut by an AWJ, so that, although more volume is removed, the removal path is not as precisely defined as is needed in most applications. The irregularity of the slot shape can be seen where a cavitating jet is traversed over a 2-inch long block of dolomite over a period of five minutes.

Figure 3. Traverse path of a cavitating jet eroding a slot into dolomite. The block is roughly 2 inches long.

Clearly, at this stage in its development, there is little precision to the slot that is being generated in the rock, and it has little application in machining.

However the operating jet pressures of these cuts are within the range of those pumps that are available at relatively low cost at hardware stores, with the simplicity in use that this implies. And yet they are capable of disrupting, into the constituent grains of mineral, even the hardest of rocks that will be encountered. Gold ore, for example, can be relatively expensive to drill, because of its strength and toughness, and yet it can be penetrated in the same way, and with the constituent minerals separated, as was the dolomite.

Figure 4. Cavitation erosion of a sample of gold ore.

The problem comes in collecting the very fine grains of the mineral from the rest of the ore sample, once it is disaggregated.

Cavitation therefore, at present, is a tool better used in applications beyond those of the machine shop. It does not, however, have to stay limited to that restriction.

Waterjetting 28c - using steel as an abrasive

One of the considerable disadvantages in using garnet and other similar minerals as a cutting agent in abrasive waterjet cutting is that the particles fragment during the mixing process, and when they hit the target. As a result (as noted in the last post) less than 50% and often much less than that can be usefully recycled. The distinction in adding the word usefully relates to the need to remove the finer particles from the recycled stock since that does not cut very well.

But what if we used an abrasive that is not degraded in the mixing process, and further one that can be more easily separated from the cuttings and spent water? The candidate is steel, which can be formed into small particles that do not degrade in size as they move through the mixing chamber, and generally hold shape even after they have hit the target. Steel also has the advantage that it can be magnetically removed from the jet stream as the flow is collected, and with no significant degradation in size it can then be readily recycled. In cases where we have monitored the recyclability of steel shot, we were able to re-use it more than fourteen times without seeing any degradation in performance. Re-using it this many times more than offsets the increased price of the original material, and will, in a short time, also pay for the relatively low costs of a magnetic separator.

Unfortunately it is not quite that simple a choice. There are a number of other considerations, which must be addressed to make the system work effectively, some of which may make the process too expensive. Three of the areas that need to be addressed form the subject of this post.

The first comes about as a result of the shape of the particles, and their retained mass and velocity on leaving the focusing tube. More than most other abrasives steel retains some elasticity during the cutting to the point that where the cutting and rebounding streams are not carefully confined, the particles can escape upwards into the cutting room. Once in the air they move at high speed, and bounce around the room, so that they can reach unanticipated places and can also be a hazard to folk doing the work.

Figure 1. Slot depths cut into granite by steel shot (left) and garnet (right)

The second problem relates to the cutting effectiveness. When cutting a brittle material the steel shot has a number of advantages, since the energy on impact is focused in the very small volume of the sphere in contact with the target. This improves the ability of the shot to generate and grow cracks in the impact zone and thereby improves the performance of the cut, over that of the mineral abrasives.

Figure 2. Relative performance of steel over garnet and sand in cutting dolomite, under otherwise similar conditions

However, when cutting ductile materials, such as metal, steel shot is not a good tool, since the focusing of the force means that the shot may get buried or just rebound from the target, without the tearing and plowing action that comes with the use of a more regular abrasive. One way to overcome the problem is to switch from a steel shot to steel grit, which is also available. The relative benefit can be illustrated by using the change from using glass spheres to using them after they have been broken into sharp fragments.

Figure 3. Effect of change in particle shape when using glass particles in cutting ductile composite material (after Faber and Oweinah).

This, by itself may not be a complete answer, since the process of making the grit makes it a little more vulnerable to abrasion and wear during the cutting process, but we have seen that it is possible to recycle most of the abrasive a number of times. However, because of the change in shape, it becomes a little more difficult to feed the abrasive into the cutting stream, and there have been occasions where the grit has bound up in the feed tube. This has, therefore, to be sized and the flow path designed, to ensure that this doesn’t happen.

Figure 4. Cuts made into tool steel using steel shot (left) and garnet (right)

The other change is to use a harder steel than normal. And here please note that there is a difference between the hardness of the steel and its toughness. As American Cutting Edge notes:

Hardness vs. Toughness: Generally as hardness increases, toughness decreases. Toughness is desirable when blades are heavily impacted, hardness when a blade is exposed to corrosive or abrasive materials.

Hardness is related to the amount of carbon in steel. Often the lower the carbon, the higher the toughness. Also, some steels do not perform at lower hardness as they were designed for use at higher hardness. . . . . . . . Hardness is a characteristic of a solid material expressing its resistance to permanent deformation. The Rockwell or Vickers hardness scales are most commonly used in the industrial blade industry.

Toughness on the other hand is the maximum amount of energy a material can absorb before fracturing, which is different than the amount of force that can be applied. Toughness tends to be small for brittle materials, because it is elastic and plastic deformations that allow materials to absorb large amounts of energy.

In general where the grit is being used to cut into other metals (which can include steels) the hardness of the cutting abrasive should be considerably higher than that of the target material.

Which comes to the third consideration, which is that steel abrasive can rust, and therefore, immediately after it has been recovered and washed, it should be effectively dried. This has proved to be more difficult to manage than originally anticipated, since, particularly where the particles are then stored for some time before re-use, any moisture present can create enough rust to “glue” the particles together. Which renders them effectively useless for further recycling and additional use.

So there are considerable pitfalls that can arise in making use of steel as a cutting abrasive, but where the jobs exist where it does effectively cut significantly better than the alternative (say in rock-cutting applications) and where the cutting zone can be shielded, and the particles rapidly recovered, dried and stored for relatively rapid recycling at an economic price, then it can be a productive way of reducing cost, while improving throughput. (And lest you think this is a new idea Gulf Oil did extensive work on abrasive jet drilling of oilwells starting in the mid-sixties, with some favorable results, but that is another story).

Waterjetting 14e - Cavitation and Comminution

In the last post on this subject I discussed how, by adjusting the back pressure in the relatively stationary fluid surrounding a high-speed jet of water, it is possible to intensify cavitation damage. The simple way to find the optimal value for the back pressure for a given jet pressure and size was, we found, to listen to the sound of the cavitation collapse, and by adjusting the back pressure tweek both the sound and range of the cavitation cloud surrounding the jet.

The damage that the cavitation would induce on samples of rock is a function of the time that the cloud plays on the surface. By slowly moving the sample under the nozzle, in a confined cell, different levels of damage could be achieved, based on the speed at which the sample moved.

Figure 1. Traversing specimen cell, with the front cover removed to show the sample in the holder.

When the sample was moved under the nozzle at 2-inches a minute, the cavitation cloud attacked the surface relatively uniformly, with only localized increases in damage. In Figure 2 the red lines mark the width of the cavitation cloud on impact, it then spreads and collapses over the surface to give the wider erosion path.

Figure 2. Traverse over the surface of a dolomite sample at 2-inches a minute. The red lines define the width of the jet. Note the additional depth of removal under the sample ID (15) where the ink chemical had slightly weakened the rock making it more susceptible to erosion.

As the speed of the sample movement is slowed, however, the cavitation attack starts to find weakness planes in the rock and preferentially begins to erode these. As these channels are formed so the jet will flow into them to escape from the following flow of water in the consequent jet flow. As the cloud moves into these narrower spaces, so the pressure increases, inducing more of the bubbles to collapse and thus intensifying the erosion attack along that weakness plane (Figure 3).

Figure 3. Traverse of a cavitating jet over dolomite at a speed of 0.5-inches per minute. Note how the jet is now eating into zones of weakness which are beginning to define pieces of rock that are then liberated as cracks grow all around them.

As the traverse speed is further reduced to 0.4-inches per minute the erosion pattern which is developing in Figure 2 becomes consistent under the full width of the cavitation cloud, and the intersection of developing cracks means that the rock is now being removed in larger pieces and the erosion rate suddenly increases significantly.

Figure 4. Effects of moving the cavitating jet over the rock at 0.4-inches per minute. The cavitation is now developing cracks in the rock that join and break out larger pieces of rock, to a depth of around 0.5 inches over the cloud width.

This ability to focus the jet attack on weaknesses in the rock structure can be useful if, for example, the rock under attack is a mineral ore. Because the ore is defined with weakness planes around the individual constituent grains of the minerals and host rock, at a slow traverse speed the cavitation cloud will preferentially attack those boundaries, in the process liberating the individual grains, and separating the rock into its constituent materials. This liberation can be achieved as the rock is being mined (we have demonstrated this in the lab) so that the valuable mineral can be separated from the waste rock at the mining machine. This means that the waste can be left, in a larger size range than is conventionally left after separation, at the mining site, and does not have to be transported to the surface and ground to powder in order to separate out the valuable minerals. The energy savings that this achieves can be potentially as high as 75% of the total energy currently used at the mine.

Where the mining breaks out the rock without achieving complete liberation a secondary process can be used where the particles of material are fed into a secondary tube, where the particles pass through a second cavitation cloud. The attack of the very small bubbles on the mineral particles is such that the fragments of ore are rapidly broken (comminuted) into much smaller sizes in a process which, because there are so many events occurring sequentially , can appear almost instantaneous.

Tests at Missouri University of Science and Technology, for example, have shown that 0.5-inch sized pieces of coal can be reduced to 5-micron size in a single step. There is a video attached to this post of one of these tests. Figure 5 shows the equipment, with the coal in the inner metal tube, while the surrounding space is filled with water under slight pressure. 

Figure 5. Equipment to comminute coal to 5-microns (The size of the feed coal can be seen in the plastic box on the right).

Water to the cell does not have to be at any great pressure. The test has been successfully run with the water fed from a pressure washer obtained from the local hardware store for less than $100.

Figure 5a. Early in the test the water flowing out of the inner tube is filled with fine particles of coal as the cavitation breaks the pieces down to the required size.

B) A short while later and the outer tube begins to fill with the fine material.

One of the advantages of coal at 5-microns is that it can be mixed with water in about a 50% slurry and fed into a diesel engine, which will then run. GE has tested a locomotive and shown that it is possible to run the engine on the mixture, should conventional diesel no longer be economically available.

The process also works when a harder rock, such as dolomite (a host for galena and other minerals) is placed in the inner tube. The cloud color in this case is white.

Figure 6. Using cavitation to crush dolomite. The original particle sizes are in the box on the left, the cloud of particles is at around 5-microns.