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.

Waterjetting 30d - Applying cavitation damage

Much of the work that we carry out with high pressure waterjets requires that they cut with precision and, in consequence much of the focus has been on controlling the stream of the jet to obtain the tightly constrained cutting action that produces this result.

Yet two of the larger growth sectors of the industry, the sub-divisions that have now been given the titles “hydro-demolition” and “hydro-excavation” don’t have that focus. Rather they seek to remove critical volumes of material, generally to some specific depth, but with less concern over the edges of the hole that is being created (provided water doesn’t penetrate the edge materials).

Depth of cut control is a little more of a challenge using an abrasive waterjet system since I have seen AWJ cuts that penetrated through feet of reinforced concrete and have mentioned the problem that new owners of systems sometimes run into when they run the nozzle for too long in a fixed position over a target and discover that the jet has not only cut the material, but also penetrated through the bottom of the holding tank, and put a hole into the underlying concrete floor.

Precisely controlling depth then becomes a matter of controlling the length of time the jet cuts on a surface, and to get to a fixed depth that will also depend on the amount of abrasive in the water, the jet pressure and the distance from the nozzle to the surface. It can also, to a degree, be controlled by the pressure of the surrounding fluid, although that is an interaction with the driving pressure that can become a little more complex.

In the last post I mentioned that when cavitation is formed around the outside of a jet cutting down through water which is itself pressurized (perhaps only because the jet is under a significant depth as water, such as for example a diver cutting apart an oil platform in the North Sea) then the damage from the cavitation bubble collapse occurs most intensely over a short distance from the nozzle. That distance changes with the cavitation number (simplistically the ratio of the pressure in the water around the jet to the pressure driving the jet itself), the volume flow and in a secondary relationship to the surrounding fluid pressure as well as other factors.

The latter impact of chamber pressure on the cutting range of the jet can be demonstrated with a Lichtarowicz cell, which allows one to see the jet as it cuts through surrounding fluid to the jet, and where, by adjusting the chamber fluid pressure the jet and cavitation cloud length can be extended to and beyond the sample, or reduced so that the jet barely reaches the target.

Figure 1. Backlit picture showing the cavitation bubbles forming and hitting the target.

The problem with generating this type of cavitation cloud as a means of drilling forward is that the bubbles are on the outside of the jet, and so as the jet hits and flows across the surface it protects the surface from the bubbles which flow on the outside of the lateral action.

The bubbles need to be confined against the target surface, and this is easier to do where the bubbles are formed in the center of the jet. The ways of doing this were discussed in an earlier post but can be summarized as being either by creating a turbulent swirl in the jet, or by placing a flat-ended probe into the jet stream.

Figure 2. Methods of creating cavitation bubbles in the center of a jet. (After Johnson et al)

Of these two methods, that using the central probe is more effective over greater distances, since the jet remains relatively coherent, while the swirling jet tends to broaden and lose energy after much shorter distances.

Tests of the central probe device showed that it could very quickly drill a hole more than 18 inches deep – at which point, unfortunately, the probe within the nozzle was itself destroyed by the cavitation action.

These tests were, however, carried out with nozzles with orifice diameters on the order of 0.04 inches, with the probe diameter being roughly half of that. Such designs are difficult to make and then align – ensuring that the probe is centered within the orifice throat, as shown.

In contrast with abrasive waterjet damage, the damage from an individual event is not as critically affected by the particle size nor by the main jet velocity. The collapsing pressure jet from a cavity collapse is at around 1 million psi – as Dr. Al Ellis theorized and we were able to confirm at Missouri S&T. This occurs with relatively little control by the surrounding fluid, or originating jet (which instead is more influential in controlling the intensity of cavitation generation and the location of the collapse).

This means that it is quite possible to use larger jet streams and still achieve quite destructive effects. In Johnson’s early paper on the topic he was using a jet pressure of 1,600 psi and able to drill through blocks of granite. The best advance rate that he could achieve at that time was around 3.5 inches/hour – which is not a practical value for commercial operations.

And unfortunately, for a while, this led us to be distracted into seeking higher and higher operating pressures to drive the jet, forgetting that this did not really change the bubble collapse pressure. It was only later, when we followed Dr. Lichtarowicz’ advice that we started adjusting the back pressure in the system and then we began to achieve useful material removal rates (on the order of cubic inches per minute).

However we did not carry out tests at larger flow rates, where we know, from the evidence at the Tarbela High Dam that much greater volumes of material may be removed, even at relatively low operating pressures.

At the Boulder Dam in the United States cavitation generated a cavity some 100 ft long and roughly 25 ft wide cutting into the rock wall to a depth of 40 ft. along the spillway during the course of a season, as reported by Warnock.

As a result of these tests it is clear that there is a considerable development potential for the practical use of cavitation – at significantly higher production rates than achieved to date, and over the wide spectrum of minerals (since the high destructive pressures exceed those necessary to disintegrate all natural materials).

It will be interesting to see when interest in the topic regenerates.

Johnson, Kohl, Thiruvengadam and Conn “Tunneling, Fracturing, Drilling and Mining with High-Speed Waterjets Utilizing Cavitation Damage.” First ISJCT
Benjamin T.B. and Ellis A.T. “The Collapse of Cavitation Bubbles and the Pressures Thereby Produced against Solid Boundaries,’ Proc. Royal Society (London), A262, pp.221-240.
Wanock J.E. “Experiences in the Bureau of Reclamation,” Cavitation in Hydraulic Structures – a Symposium, ASCE vol 71, no 7, p 1053. (Sept. 1945)

Waterjetting 30b - An opportunity missed, and a question raised

In the last post I wrote about the benefits of cutting a deep slot around the edges of a tunnel, and made reference to the work done here by Dr. El-Saie back in the mid-1970’s as part of his Doctoral Dissertation.

One of the concerns that we had to address was that the waterjet had to be able to penetrate all the different rock types that it might encounter, and at the same time, since the jet would only cut a short depth on each pass we also had to find a way of cutting the slot wide enough that the nozzle assembly could enter and deepen the slot over consecutive passes around the edge. Given that the available pressures in those days were limited, for us, to 30,000 psi and this pressure was insufficient, by itself, to cut through all the rocks we might encounter, Dr. El-Saie looked at several different ways of enhancing performance. These included adding abrasive to a high pressure jet stream, inducing cavitation into the jet stream and the potential for using the break-up of the jet into droplets to enhance cutting using the impact water hammer effect.

Because of other operating conditions it was not considered practical to try and develop the droplet impact idea for this program, and the work concentrated on examining the potential differences between abrasive waterjet injection and cavitation. To simplify the comparison the same basic nozzle design was used for the tests that were then run, although the shroud fitted to create the secondary (vacuum) chamber was modified to either allow abrasive entrainment, through ports, or to create cavitation. The presence of the ports did, however, allow the strength of the vacuum generated in the chamber to be measured as the jet passed through.

Figure 1. Nozzle designs used by Dr. El-Saie. Note that the upper design has ports leading into the vacuum chamber, so that abrasive can be drawn in by the jet passage. In the lower design there are no ports, and cavitation will be induced in the chamber by the jet passage, with the bubbles then drawn into the exiting jet.

One of the advantages of cavitating the jets is that the cavitation bubble collapse will spread out over a larger area on the target surface, so that the slot generated can be quite a bit larger than the originating jet. This can be shown in two pictures of a block of dolomite exposed to the same cavitating jet, at a pressure of 6,000 psi but one with the jet traversed along the block in a minute, while in the second case the jet is moved at a slower speed, taking five minutes to cross the block, which allows the jet to exploit the cracks generated by the cavitation. A fuller description of the process is given here.

Figure 2. Cavitation damage pattern on a block of dolomite showing the initial width of the jet (red lines), and the zone of damage that is being created around the traverse path.

Figure 3. Cavitation damage on a block of dolomite at a slower traverse speed, showing the width of the damage track that can be created. The slot is about half-an-inch deep.

In the course of the test program different shroud shapes were tested, but in all cases the comparison between an abrasive-laden jet and one containing cavitation bubbles was made with shroud shapes of the same overall dimensions.

The ratio of the exit diameter (discharge) from the shroud (D2) to that of the initial jet orifice (D1) was first changed to one of four different ratios, though the diameter of the initial jet was kept at 0.04 inches (1 mm). Of the different sizes tested the greatest vacuum in the chamber was measured with the smallest of the discharge diameters was tested.

Figure 4. The effect of increasing the throat length of the shroud on the vacuum pilled in the chamber, at different pressures.

If the discharge diameter was increased to 6.35 mm then the jet pressure had to be increased to 12,500 psi to obtain the same levels of vacuum achieved otherwise at 7,500 psi.

Figure 5. Vacuum pressures measured with a larger discharge diameter from the shroud, for different lengths and jet pressures.

Impact force measurements from the jet hitting a target at varying distances from the orifice, with and without the shroud showed relatively little difference in the overall total impact force (not considering abrasive) out to a distance of 6 inches. There was thus no apparent effect due to jet disintegration from the use of the shroud over these distances.

In order to compare the performance of the abrasive-laden jet with that of a cavitating jet, a new nozzle design was developed, and small samples of granite were rotated in front of each nozzle assembly, for 20 seconds. Because the jet had to be brought up to pressure for each test, and shut down afterwards, a steel shutter plate was placed between the nozzle and the target. One of the irritants in doing the tests was that the jets kept cutting through this shutter plate.

Figure 6. Steel shutter plate cut through in 6 seconds during system start-up.

The shroud, made of stainless steel, was also wearing out within a few minutes. Unfortunately we did not recognize that this was demonstrating that abrasive waterjets were an effective method for cutting metal – that commercial development had to wait for the more perspicacious Dr. Hashish to work with Flow Research and bring the technology to the market in 1980.

Part of the reason for our lack of interest was because of a different conclusion that Dr. El-Saie drew from his work, based on the following two curves. The first comparison of different jet results occurred with a jet pressure of 7,000 psi.

Figure 7. Volume of material removed from granite samples, as a function of distance, for four different jet conditions at a jet pressure of 7,000 psi.

Note that the three water jets do not have much significant effect on the granite at this distance and jet pressure (we had to learn some later lessons to make them more productive at this pressure). But even at this pressure the abrasive waterjet was effectively cutting the granite.

But it was the change in the relative position of these curves, as the pressure was then increased to 20,000 psi that caught our attention. (The intermediate plots are not given here).

Figure 8. Volume of material removed from granite samples, as a function of distance, for four different jet conditions at a jet pressure of 20,000 psi.

The water feed was not useful, since the power required to accelerate that volume drew heavily from that available through the jet.

The plain jet, without a shroud will cut granite at this pressure, particularly when moved over the surface. (And we later used this system to carve the Millennium Arch_ – as well as the Missouri Stonehenge). But the performance at that time was not that impressive.

Opening the ports on the shroud, without feeding anything into the jet caused, we believed a greater jet breakup and thus some additional droplet impact effects that improved cutting performance over that of the plain, more coherent jet, in part because the jet was spread over a larger contact surface.

Closing the jets induced cavitation in the stream, and this gave the best performance of the four – including abrasive injection. Again this was, in part because of the larger area of damage that the cavitation generated on the target, over the narrower slot of the abrasive-laden and plain jets.

In comparison the abrasive waterjet did rather poorly. In retrospect this is perhaps more of a surprise – but it should be born in mind that there was little attempt at optimizing the feed condition (which later research shows has a dramatic effect on performance) or the chamber geometry. Further the slot cut was much narrower than that created by the cavitating jet.

But it certainly caused us, in that time interval, to look more at cavitation, and to totally miss the implications of the AWJ result.

Most of these illustrations come from the Doctoral Dissertation by Dr. A. A. El-Saie “Investigation of Rock Slotting by High Pressure Waterjet for Use in Tunneling”, Mining Engineering Department, Missouri University of Science and Technology, (Then University of Missouri-Rolla), 1977.

Waterjetting Technology - Dr. Andrej Lichtarowicz

I was saddened, this week, to hear of the passing of Dr. Andrej Lichtarowicz, who died on the the 6th of this month. As Mark Fairhurst noted, Dr. Lichtarowicz, on the faculty at Nottingham University, was a sterling early contributor to the waterjetting community, from its birth back in the early 1970’s. He gave a paper at the first BHRA conference in 1972, and serving as the editor of the 11th Proceedings in 1992.

There were 201 delegates to that first meeting, which was held at the University of Warwick, in Coventry, UK. with 37 papers being given over the course of two and a half days. The use of high pressure waterjet technology was very new at the time, and this was the first time I was able to get together with peer scientists from around the world to discuss what they were doing as well as make a small contribution of our own. But the papers that influenced our lab the most were the two given on cavitation. (One was given by Dr. Andrew Conn and the other by Dr. Lichtarowicz). The reason for this interest was that back in those days the pressures available from high-pressure pumps were restricted to about 30,000 psi, and (without abrasives which only showed up about eight years later) this significantly limited what materials could be cut.

Dr. Conn’s paper related to cavitation at lower pressures and higher volume flow rates, while Dr. Lichtarowicz’ paper covered smaller jet testing at pressures of up to 10,000 psi. Using such a jet he had been able to drill holes in aluminum, which he could not do when the jet was not cavitating. The results were sufficient that we shortly thereafter tried to repeat , and were able to exceed, these results, drilling a hole in a small piece of alumina in less that a minute, although at a pressure of around 18,000 psi.

This led to considerable discussion at the 2nd conference, which was held in Cambridge in 1974, as to whether the results that were being reported were because the jet was breaking up into droplets, or if the result was true cavitation. It was a discussion that Dr. Lichtarowicz, as always, took a significant part in, and although he could not make the third conference (which was in Chicago) by the time of the fourth, in Canterbury in 1978, he was carrying out his research with the nozzle and target submerged with enhanced results from the earlier work.

Over time he developed a small cell, with windows so that the action of the jet could be seen.

Figure 1. Initial design of the Lichtarowicz Cell

The small size of the unit, and the relative simplicity of construction, meant that a number of us, around the world, built such units and used them to help develop a better understanding of what was happening, and how damage could be increased.

One of the early discoveries he made was that, by adjusting the pressure in the chamber, the amount of overall damage (measured by mass loss) could be significantly intensified, and the rate of erosion increased. It was on that basis that we, among others, were able to use cavitating jets to disaggregate rock and coal into fine particles.

Figure 2. View through the port of a Lichtarowicz cell, showing the cavitating jet impacting a metal target.

One of the major uses of the cell was, however, not as a tool to develop faster ways of drilling rock (though it did) but instead to accelerate the rate at which the cavitation resistance of different materials could be determined. Until that time the standard tool for determining cavitation resistance had been the vibrating horn device recommended by ASTM. The problem with this was that it took hours (typically about 24) to generate the data and plot the rate of material removal, because it was so slow. With the cell a similar result could be obtained in minutes. His work led to the development of an ASTM standard first adopted in 1995, and reapproved in 2001 and 2006, with current interest in revision. It went on to be incorporated as part of the International Cavitation Erosion Test.

And so the technology moved forward, Dr. Lichtarowicz gave his last BHR paper at the 12th Conference in Rouen in 1994, and this was a review of some of his earlier work, showing its relevance as industry sought to find cleaner, greener methods for cleaning and material removal. His fundamental work, however, fostered studies that continue to live on, particularly in Japan, where laboratories continue to develop the techniques and ideas that he pioneered over the years. Certainly our own work would not have progressed as far, or in as many directions, without the inspiration of his work, and the many discussions on the technology we held over the years.

He was a good friend, not only to young faculty – as I was when we first met – but to the industry as a whole, and the students that he taught over the years. He was a much respected scientist and colleague and the tools that he developed and helped us learn to use will continue.

Waterjetting 27e - Borehole Back Pressure Effects

In the earlier posts on this chapter of waterjet technology I have dealt with the changes in cutting performance when a waterjet stream cuts in to material that is either under pressure, or contains internal stresses that may not be obvious at first glance. In this post I will focus, instead, on the changes in performance when the borehole becomes filled with water under pressure.

Figure 1. 12-inch cores of sandstone that have been drilled by the same jet drill, at the same speed, but at borehole pressures of 0, 500 psi, 1,000 psi, 1,500 psi and 2,000 psi. (Jet pump pressure 10,000 psi; 970 rpm; 40 inches/min ROP)

The water used in the test also contained a small amount of polyethylene oxide (Polyox) that, at the time, was the only polymer readily available to enhance jet performance under water, although there are now liquids such as Superwater that similarly help.

It can be seen that even the change in pressure to 500 psi is sufficient to dramatically shorten the distance that the jet cuts through the material on a single pass, and the range then only shortens a little as pressure further increases. But the hole drilled at 2,000 psi is barely large enough to let the high pressure lance and nozzle assembly pass.

First an explanation of the equipment that we used to run the tests. A triaxial cell was used as the basic vessel to hold the core. This is so-called since it allows pressure to be applied around the rock core, and also since the cap can slide within seals, axial pressure given the third of the orthogonal directions for loading.

Figure 2. Triaxial cell used for the drilling experiments.

A valve was fitted on the flow line of water out of the chamber (just above the pressure dial) and this controlled the fluid pressure in the cell. The diameter of the outer (reaming) jet was 0.04 inches, and the rapid decay in range with the increase in pressure led to a second experiment, to see how changing the diameter changed the results. The equipment was modified for this test, the feed pipe to the nozzle was bent, so that, as it made a single circuit over the underlying rock, it would trace out a circular path rather cut a single hole. Then the top of the sample was cut at an angle so that, with the rotation the distance from the jet to the target would vary and the range of the jet could be seen. (Figure 4).

Figure 3. Modified equipment to find the effective jet range against back pressure.

A simplified factorial experiment was run with three nozzle diameters and five back pressures, measuring the depth of cut into the sandstone in each case.

Figure 4. The resulting cut when a 0.03 inch diameter jet was rotated over sandstone with a 1,000 psi back pressure in the cell. The 10,000 psi jet was brought up to pressure with the jet at the greatest standoff (hole at the bottom) and the back pressure was set before making a single pass over the sample. The depth of cut was averaged over several readings made along its length.

The data was then plotted (with the curve smoothed here for simplicity in discussion).

Figure 5. A plot of range of jet cutting ability as a function of hole back pressure for three different nozzle diameters.

The graph shows that, for this set of conditions, the larger the jet the better, and that the first 500 psi of back pressure has an immediate effect on jet cutting effectiveness. Jet size should be at least 0.064 inches when drilling against back pressure in the hole. There was a significant improvement in cutting ability when the polymer (at 300 ppm) was subsequently added to the water, in a later series of tests. The small number of tests carried out, however, were too small a sample to provide more than guidance as to concentration since all three levels tested (100, 200 and 300 ppm) all showed considerably improved depths of cut (increasing to a depth of almost 2 inches against a back pressure of 2,500 psi) when contrasted with the performance levels shown above. The polymer tests were carried out with a jet nozzle diameter of 0.064 inches.

There are two parts to the effect of the borehole pressure. The first is simply one of increasing the resistance of the water to jet penetration, and lowering the effective jet pressure (since that is effectively the jet pressure less the borehole pressure).

It is important to recognize that it is not just the drop in effective pressure that causes the effect. To check that this was the case a hole was drilled with the same conditions otherwise as the left-hand rock sample in Figure 1, except that the jet pressure was dropped to 5,000 psi. Thus the differential pressure of the jet across the nozzle was less than that in the case of the other four rock samples shown in Figure 1. Yet the hole was of the same approximate irregular geometry as that shown by the left-hand core of Figure 1 even with the lower differential pressure with the prominent cone cut ahead of the bit that is not evident in the other cases.

Mike Hood has shown the effect of loss in cutting range by using back-lit shadow images of a jet at different back pressures.

Figure 6. Illustration of the effect of fluid back pressure, the shadow image of the jet shows how back pressure reduces the range.

As mentioned above, the effects extend beyond reducing the jet range, and lowering the jet differential pressure. The increased confinement on the rock will compress the grains of the rock more tightly together, making it more difficult for the pressurized water to penetrate into the rock structure. This combines with the higher pressure required to grow the cracks to effectively reduce the ability of the jet to penetrate into the rock.

At the same time, if you listen as the back pressure is increased (we used a Lichtarowicz Cell the increasing pitch of the sound shows (as does the damage induced) that the collapse of the cavitation bubbles generated around the edges of the submerged jet is becoming more intense as the pressure increases. I have discussed how this can be used as a benefit in breaking up rock in an earlier post.

Tech Talk - a gentle cough for Simon Michaux

In response to a post I wrote a couple of weeks ago Marty suggested that I watch a video by Simon Michaux discussing peak mining. So I did.

A quick check through Linked In has Simon Michaux as a Mining Consultant in the Brisbane area, having been a Senior Research Fellow at the Julius Kruttschnitt Mineral Research Center (JKMRC) at the University of Queensland for 14 years. (I should mention I spent a sabbatical at the Mining Department at the University of Queensland in 1987).

In his presentation Simon discusses Sustainability in regard to the mining industry, pointing out that as the population rises and demand for minerals increases, that demand can only be met by mining leaner and deeper ores, once the shallow easy and cheap to mine deposits are gone. (A similar argument to that of peak oil, which he does talk about in his presentation as well as mentioning the predictions that we are nearly at Peak Coal).

I have a number of problems with his approach, and have discussed some of them in various posts over the past few years, but let me discuss them again as a rebuttal to his conclusion that the world is rapidly heading into disaster and the end of the Industrial Age as the costs to mine minerals and the difficulties in finding enough product make it impossible to continue our current trends.

Now it is true that back in the days when Europeans first came to the United States that the local tribes around the Great Lakes were mining pure copper strips and large slabs and nuggets could be found. White Pine Copper Mine in Michigan was still finding these when I visited there some decades ago, but they occurred in a relatively hard host rock and the deposit was going deeper and becoming more expensive and so the mine closed. Because of economies of scale it became cheaper to simply dig much lower grade ores out of the ground. He cites the example of Bingham Canyon where the mine now extracts copper from ores with less than 1% of disseminated copper, rather than the pure copper nuggets of former times. And he points out that as the ore is ground finer it requires more power.

Figure 1. Relationship between energy required to liberate minerals from ore by reducing the particle size, leading to higher energy demands. (Simon Michaux)

There are a couple of points that need to be raised here. The first is that digging ore (and coal) out of a surface mine is a relatively simple and comparatively inexpensive operation. It does not require large applications of exotic technology and the whole process of getting the ore from the solid to the point where the mineral is liberated is straightforward.

The reason that there are steel balls shown on the rhs of the above figure is that after the body of the ore is broken free with explosives the fragments are picked up in a large shovel and loaded into mine trucks that carry hundreds of tons at a time to the main plant where the ore is crushed in part by falling into long rotating drums filled with steel balls that break the rock into fine particles through impact and attrition. (A simplified modern version of pounding the rock fragments with hammers until it gets small enough to free the mineral). Simon makes the point that modern technology is now capable of breaking the ore down to 5 micron particle sizes to free the ore, but that this takes increasing amounts of energy (as shown above), and that hauling all the ore to the plant and crushing it all to this small particle size is leading to unsustainable energy costs – particularly as oil and other fuel prices are set to continuously rise in the future.

But here he makes a critical misjudgment, because his argument rests on the mining industry and the manufacturing industry remaining the same, and following conventional practices into the sunset. But this is unlikely to happen. Just as the increase in prices made it possible to develop hydrofracking of long horizontal wells and thereby develop the oil and gas in the otherwise uneconomic deposits of Dakota and Pennsylvania so technology can find alternate processes that will lower the costs for mining minerals.

For example it is not necessary that the trucks that haul the ore rely on diesel fuel produced from oilwells. Some mines have already switched to biodiesel, which has some advantageous properties for their operations. Other mines use electrical power to run their haulage and GE has demonstrated that diesel engines can run on a mixture of fine coal and water. The reason that countries such as the UK have migrated away from coal use has more to do with the availability of cheaper sources of alternate fuel and for political reasons rather than there being a lack of available coal. (Note that German use of coal for power is increasing as an example).

Secondly the use of ball mills for crushing all the ore is simple but not necessarily all that efficient. I have noted that a more efficient process, wherein ore can be reduced in one step from 1 cm size to 5 micron size, using cavitation, is quite easy to build and operate.

The use of hydroexcavation and instant ore comminution using cavitation means that the ore can be separated into mineral and waste at the mining machine, and (because of the way the process works) both fragments of the ore are broken at the natural grain size, so that there is no need for overgrinding, and the fragmentation is by tensile fracture growth instead of compressive crushing, saving energy. By separating the mineral at the face, and leaving the waste in larger fragment sizes the waste can be relocated close to the mining face, potentially being used to provide support in regions that have been mined out. Only the mineral needs to be moved from the face to the plant – cutting energy costs dramatically.

Once the mineral is available as a fine particle it becomes easier to treat it and process it into the required feedstock which, as 3D Printer technology migrates into the construction of larger and more useful items from metals and more advanced materials so the waste involved in older conventional practice will be minimized and costs in financial and material items contained.

The future is likely therefore to be much more exciting and positive than Simon Michaux foresees, though I do agree that it will become more sensible to mine landfills to reclaim minerals – but then we have been doing that for some time now. But no, we are not coming to the end of the Industrial Revolution, merely moving to a different phase.

Waterjetting 15d – More thoughts on cut surface quality.

When a high-pressure stream of water hits a surface, the arrival of subsequent lengths of the waterjet stream forces the initial water away from the initial impact point, into and along any weakness planes in the target material. As a result there is some preferential cutting of the material, especially where there are defined weakness planes in the material. One illustration of this is where a jet that contains cavitation bubbles impacts on a rock surface (figure 1) and as the water enters the narrow eroded channels where preceding lengths of water have preferentially eroded out the weaker rock the pressure in the channel increases, collapsing the remaining cavitation bubbles and further exacerbating the damage within that narrow channel, causing it (them) to grow preferentially relative to the surrounding rock.

Figure 1. Looking down into a channel cut by a cavitating jet that traversed from left to right, at a speed of 0.4 inches/minute. Note the preferential attack into weakness planes within the rock.

As the weakness planes grow and join, so individually larger pieces of rock can be broken free from the target and the path, and pressure profiles of the water in the cutting zone change quite significantly. For this cavitation to have a significant impact on the erosion pattern, however, the traverse speed over the surface must be controlled, and be relatively low. At more effective speeds the cutting process does not allow for the development of this fracture mechanism. Rather, with plain jets, the process concentrates just on crack growth around individual grains. Optimum cutting speeds are much higher, depending on the intended result.

The efficiency of waterjet cutting has, historically, been assessed in terms of how much energy is required to remove unit volume of material. This we call the specific energy of the cutting process, and a common unit is joules/cubic centimeter (j/cc). When using a waterjet to cut into material, in part because of the interference between different segments of the jet stream, pre and post impact, the most efficient cutting speeds are quite high.

Figure 2. The change in cutting efficiency with traverse speed of a high-pressure waterjet cutting stream

The downside to using higher cutting speeds (apart from the simple inertial problems in driving systems at higher speeds in other than straight lines) is that the depths of cut achieved become smaller on individual passes, as the jet has less cutting time on each path increment.

Figure 3. Change in cut depth as a function of traverse speed, for varying different rock types.

In linear cutting systems it is sometimes possible to align secondary or a higher multiple array of nozzles along the cut, so that thicker materials can be cut with a sequence of jet cuts along the same path. Alternately a single nozzle can make multiple passes along the cut path and sequentially deepen the slot.

Unfortunately while this is an effective way of solving some problems, it becomes less efficient as the slot gets deeper.

Figure 4. The change in cutting efficiency with increase in the number of cutting passes.

At higher pass numbers with the target surface at a growing distance from the nozzle, and with the edges of the cut starting to interfere with the free passage of the jet to the bottom of the cut, less energy is arriving at the bottom of the slot and thus the effectiveness falls.

While there are differences between abrasive waterjet cutting (where the optimal cutting speed is much lower than that for a plain high-pressure water jet) the form that the cutting jet takes through the target material is of similar shape in both circumstances.

Figure 5. An abrasive waterjet cut through 1-inch thick glass

As the jet cuts through the piece, so the cutting edge curves backwards from the top of the cut to the bottom. The rate of this curvature is, inter alia, a function of how fast the nozzle is moving over the surface. Dr. Ohlsson showed this effect in cutting through 0.4-inch thick aluminum and mild steel plates, back as part of his doctorate at Lulea in 1995.

Figure 6. Change in the cutting edge profiles and cut groove patterns in metals as a function of cutting speed (L. Ohlssson PhD Lulea, 1995)

The growth of the striations in the cut surface, as the depth of cut increases is one of the larger concerns with cut surface quality, since customers are often concerned that these be minimized, and further if they become large enough they can make it difficult to separate the pieces, particularly if the parts are cut with a complex geometry.

Early in the understanding of the way in which waterjets work, it was thought that the jet would incrementally cut strips from the material in front of the previous cut, inducing steps into the cutting plane that worked their way down the material.

Figure 7. Early concept of cutting front development (L. Ohlssson PhD Lulea, 1995)

However, as higher speed cameras recorded the development of the cutting front, this concept has been rethought. Henning, for example at the 18th ISJCT, used a camera taking 520 frames per second to establish the development of the cut profile as the jet cut through clear plastic. In figure 8 the profiles are shown as they developed at 35 frames/sec to allow them to be distinguished.

Figure 8. Cutting front development as an abrasive jet cuts from right to left (Henning 18th ISJCT)

As Ohlsson had shown this profile develops as the abrasive laden jet impacts then bounces, then impacts and cuts further into the material, as it moves down the cut.

Figure 9. Frames showing a sequence as an abrasive waterjet cuts through 2-inches of glass. ((L. Ohlssson PhD Lulea, 1995)

In his work Henning correlated the change in the “bounce angle” with the jet properties, while Ohlsson also correlated with the traverse speed.

Figure 10. Change in the “bounce” angle as an abrasive jet moves down the cut (Henning 18th ISJCT)

Two things should be remembered in this analysis, since they also explain causes of the increased roughness of the cut each time the jet bounces. The first is that the jet is not only laden with any initial abrasive, but as it cuts into the material, and removes it so that cut material is entrained in the jet, so that there is some abrasive cutting, even with a plain waterjet once the initial cut has been made. The second point is that when the jet bounces it is not constrained to bounce just in the plane of the cut, but can and does take up some deflection into the sides of the cut. Thus, with each bounce and reflection, the cut becomes rougher as that side cutting becomes more pronounced.

However the number of bounces can be slowed by slowing the speed at which the nozzle moves over the surface.

Figure 11. Change in the angle along the cutting edge as the speed of cutting and the jet pressure are changed (H. Louis, Waterjet Conference, Ishinomaki, 1999)

Henning uses a different term, but nevertheless it is clear that increasing the jet pressure and changing the diameter of the jet stream also controls the edge profile, and as discussed, with a smaller number of bounces so the edge quality improves.

Figure 12. The effect of changing jet pressure and jet diameter on the gradient of the cutting edge profile (Henning 18th ISJCT)

Tech Talk - Energy cost, additive engineering and cavitation

I paid $2.85 for a gallon of gasoline this weekend, at the gas station just up the road from our house, here in South Central Missouri. A couple of weeks ago while I was in the UK the price my brother paid was around $8.00 a gallon. The BBC calculator that I used to check the UK price tells me that I am paying $6.89 less per tank than the regional average here, and that were I to live in Italy my tank-full would have cost me $95 more, while it Venezuela it would have cost $43 less. (It cost $45 to fill my tank).

The low cost of fuel is one of the benefits from the increased crude oil production in North America, sustained as it is by the increase in production from Saudi Arabia to balance the global market losses from other countries around the world. Further the EIA explains the refineries are helped with this low price by the high demand for diesel and the premium that it has achieved – causing refineries to run at record levels to meet the demand, and producing, as a secondary product, more gasoline that is thus being marketed at the lower price. It is a situation that the EIA expects to continue for a while.

Figure 1. US refinery inputs (EIA TWIP Nov 6, 2013)

The relatively low price of fuel, here in the United States, particularly relative to Europe is starting to attract industries historically located abroad. The move to date is being led by those attracted by the cheap price of natural gas, particularly in the chemical industry. BASF, for example, cut the ribbon last week on a plant expansion in Vidalia, LA and just recently announced plans to expand its research facility in Beachwood, Ohio.

It was, however, another report on manufacturing that really caught my attention this week. It was the news that 3D Printer technology had advanced enough to now make a gun from metal parts. The process involved is somewhat more complicated than that used in earlier guns manufactured using this new generation of equipment. Earlier in the year a gun had been made from plastic parts and made some additional news when a version fired nine shots without falling apart. The evolution of the plastic gun is worth noting in that the first one reported was built from components printed with an $8,000 second-hand Stratasys Dimension SST 3D printer. And while it fired a shot successfully, the gun blew up on the second trial. The second gun, however, was made on a $1,725 Lulzbot A0-101 3D printer, that was available from Amazon, made by Aleph Objects and it survived firing nine rounds. For a variety of reasons the plastic gun contained some metal parts, but it marked the advent of this new technology. Prices for these replicator units are already down below $2,000 and they are limited, at present, to working with different types of thermoplastic. (But they can make, for example, shoes.)

The difference in being able to move to making parts from metal, particularly those that allow the repeated (over 600 times) firing of the gun is a very significant step forward. Thirty-four parts were made from stainless steel and Inconel 625 and then a grip was made from nylon, using a classic 1911 design.

Figure 2. The metal gun made by Solid Concepts (Solid Concepts )

It is the different metal part of this that is worth underlining. The components were made by laser-sintering (which simplistically means that they used a laser to melt tiny particles of metal so that they would fuse together to make the model). The machine that is used to do this, at the present time costs between $400,000 and $1,000,000. It also has power and other logistic needs that require it be run in a commercial, rather than residential environment.

But, as Sold Concepts notes:

Solid Concepts has been using metal sintering for some time now to successfully create parts for a wide array of products. The 1911 gun is well known and people can relate to it in respect to its power and need for precise components. This story is about how additive manufacturing can be used to produce real, accurate parts in your industry whether it’s aerospace, transportation, medical, energy, consumer products, etc.

The changes that this will make in industrial manufacturing, and in the global market for materials cannot be underestimated. At present parts are generally made by subtraction, taking large billets of material and milling and machining away all the un-needed bits, producing large volumes of scrap chips. None of that waste will be generated with this new process.

Chris Hechtl has already produced The Wandering Engineer” series of Science Fiction books, starting with New Dawn that uses the concept widely as one of the bases for the stories. (Worth a read just to get some idea of the scope of what is to come - though I am also enjoying the series, as the books are written).

It is going to change the way in which components are built, but it will also change the way in which minerals are processed once they are mined from the earth. It will be no longer necessary to cast metals into large ingots and then forge them down into smaller shapes. It is likely that, for many items in the near future that process will still be cheaper, but as time progresses and the costs of the process reduce (bear in mind that this is laser-based and remember how those costs have come down as lasers have become ubiquitous in society) that even large parts may be better made this way. Further it allows intricate melding of different materials to make products that are stronger and better suited to the need.

Thus the objective of mineral processing in the years to come will be aimed at making fine powders rather than going through all the steps to make the larger ingots. That will, in turn, impact earlier stages of processing, and, while I don’t normally discuss my own work in these posts, I would draw your attention to a recent post from October 31st, down below, which includes a video of a small piece of equipment virtually instantly breaking half-inch coal into 5-micron pieces, which can be done with a pressure washer from the local hardware store. It also works in breaking out minerals from their host rock.

The world indeed will change, and with those changes the power requirements of the future are also going to undergo drastic revision.

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.

Waterjetting 14d - Traversing Cavitation

Within the normal range of everyday fluid flow cavitation is something to be guarded against. Because it occurs when water is put under conditions where, through geometric or working conditions, it is either pulled in tension or shear to create the small cavities of sensibly vacuum that lead to cavitation damage, there are many ways it can be formed. In an earlier post I mentioned the cases where moving water past a blunt-ended surface at high speed can cause the bubbles to form. In an alternate form, this cavitation can be generated when solid bodies are dragged through relatively stationary water at high speed.

The most common example of this is with propellers and underwater craft, where the relative flow paths over a moving, submerged body can cause bubbles to form and collapse. When the bubbles collapse, even though individually tiny, they can, because of their number, create a fair amount of noise. That noise, generated behind the spinning blades of a driving propeller, was one way in which submarines could be detected and located during the Second World War.

Figure 1. Cavitation forming as water flows around a probe.

Notice that, in Figure 1, the bubbles form and collapse over the length of the probe, which was, in this case, held stationary while water flowed over the surface within a tube. The relative motion is the same as though the probe, a potential submarine shape, was moving at speed through stationary water.

There have been a number of different flow chambers built at different research centers, each in their own way trying to build a device that would allow study of the ways in which cavitation damages surfaces, and to evaluate different materials for cavitation resistance. I have mentioned the ASTM test methods earlier.

Figure 2. German cavitation test apparatus

In one such design, (Figure 2) German investigators built a flow channel where the flow channel was narrowed, and then expanded to induce cavitation in the downstream flow. By then placing a test specimen at the point of maximum bubble collapse, a test could then evaluate the different material responses.

The concept of creating shear, as well as tension in the water around a flowing jet can similarly be imagined where the design above is modified so that the jet that issues into the downstream flow is pressurized to higher velocities. This was the basis for the cavitation cell developed by Andrej Lichtarowicz at the University of Nottingham. This can be applied in a number of ways, in the one below, for example, Canadian investigators had developed a portable version of the concept.

Figure 3. Early method for inducing cavitation around a submerged jet.

However, it was the Nottingham cell that provided the basis for a move forward in the technology as a number of us, around the world, collaborated with Dr Lichtarowicz in trying the new concept. Early on we noticed that if one listens to the noise made by such a jet, it is possible to hear a change in the pitch of the sound as the relative pressure in the surrounding water changes, relative to that of the driving jet. This relationship is defined by the definition of a value known as the Cavitation Number of the condition.

In this equation Pd is the pressure in the downstream fluid, Pv is the vapor pressure of the fluid, and Pu is the driving pressure behind the jet.

For higher pressure cavitation flows the vapor pressure is sufficiently small that the equation can be simplified to the ratio of the downstream pressure (say 50 psi) divided by the jet pressure (say 10,000 psi) which would give a close approximation to the cavitation number ( 0.005).

Dr. Lichtarowcz simplified the design of a cell in which a submerged jet could be directed at a target, with the back pressure in the cell adjusted to control both the intensity of the resulting cavitation, and also its position of maximum damage.

Figure 5. Early design of a Lichtarowcz Cavitation Cell

This design was of interest to us, since it allowed rock samples to be used and evaluated, and we built and tested several different models based on this design. The two windows allowed the jet and specimen to be lit and viewed during a test.

Figure 6. View of a cavitating jet, with the cavitation cloud of bubbles collapsing at the surface of the specimen on the right.

Dr. Hood, in Australia, has shown, with high-speed photographs, how changing the back pressure in the chamber changes the effective damage range of the jets.

Figure 8. Back-lit photographs of jet and cavitation cloud collapse as the ambient chamber pressure is increased.

The above pictures show why, in underwater applications, the range of a high-pressure waterjet becomes increasingly restricted as the pressure increases. This is of great importance where, for example, high-pressure jets are being sent to the bottom of an oilwell to clean the filter screens. The range of the jet is controlled, in part, by the jet diameter, as well as the pressure, but can also be expedient to add different chemicals to the flow in order to enhance the range, and I will write on that in a later piece.

However, it was through the control of the range, and the intensity of the cavitation that we discovered, in applying the cavitating stream to rock, that the damage was now occurring at a fast enough rate that the small samples, and longer test times of the conventional test were no longer viable. The small samples were being consumed in a very short time, and so the design was modified, so that a target block of rock could be moved under the jet, at a rate of around an inch a minute, while maintaining the cell pressure to intensify the damage.

Figure 8. Section of a 2-inch deep hole drilled (at 6,000 psi jet pressure) into a block of dolomite. The damage is caused by cavitation since, at that jet pressure, the fluid would otherwise not damage that particular rock. Note that in this test the sample was not moved relative to the nozzle, and the jet was impacting at the top of the rock, which is to the right end of the hole drilled.

Figure 9. Traversing specimen test cell – schematic view from the top showing the starting position of the jet. The jet is positioned on the end plates until the test starts, and the sample is moved under the jet until the second end plate is reached, when the test is concluded.

The sample had to be moved at a controlled speed since, as the jet cut down into the rock, so the target surface moved away from the jet, and the depth of focus for maximum cavitation damage is relatively narrow – depending on the test condition.

It is in this balance between the effective damage range of the jet, and the intensity of that damage, that is yet to defined in a way that will focus the intensity of damage to its greatest potential. However there are other ways of using that potential, and I will describe those next time.