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.