ORIGIN AND APPLICATIONS OF AWJ
The waterjet technology originally developed in the mining sector and dates back to ancient times, with documents illustrating the usage of water to facilitate the mining of valuable material in Egypt and later in Roman times. More recently, pressurized water was used in California to excavate gold during the 1853-1886 period and later on the technique was applied to coal mining in the Soviet Union during the 1930s (Summers, 2003).
A first application of water as industrial cutting device was developed in 1933 by the Paper Patents Company, in Wisconsin, which patented a machine using a waterjet to cut a sheet of paper (Fourness and Pearson, 1935). From that starting point, the water pressure (and thus the cutting capability) of waterjets continuously increased: in the 1950s, short burst of very high pressure able to cut wood were obtained (Flow International Corporation, 2014), but no commercial application of the technology was developed. This had to wait until the 1970s, when the first equipment was installed and waterjets became for the first time a viable option for the manufacturing industry (Summers, 2003).
Despite the high pressure used, the cutting capability of the pure water jet (PWJ) was still limited to a small range of softer materials. A breakthrough in this sense arrived in 1979, when Dr. Mohamed Hashish developed a nozzle which was able to combine an abrasive material (such as garnet) into the high-pressure water jet, drastically increasing its cutting power and allowing it to cut materials such as glass, steel and concrete. By 1983, the first commercial abrasive waterjet (AWJ) cutting system were sold (Flow International Corporation, 2014). Since then, the technology further developed, allowing Abrasive Waterjets (AWJ) to efficiently machine virtually any material.
AWJs present a number of important advantages that fostered the diffusion of this technology in a large number of industries, with their extreme flexibility being the most important one: as mentioned in section 1.1, AWJs can cut through almost every material (Khan and Yeakub, 2011) and they can be used on both thin and thick pieces, including stacked or multilayer materials, executing rough cuts as well as fine edge finish on complex contours; Using water jets, parts can be nested very close to each other in order to maximize material utilization, and very little material is removed from the workpiece due to the narrow kerf (Kulecki, 2002). During the cutting process, almost no heat is produced and only minimal reactive forces act on the material, thus preventing the introduction of heat affected zones or residual stresses (Hlaváček et al., 2012). The low sideway reactive forces applied to the workpiece also allow to machine honeycomb structures that would otherwise be too fragile to cut (Hunt et al., 1987). On the other hand, the technology also presents some limitations: as it travels through the workpiece, the water jet is deflected to some extent, especially affecting the accuracy in case of thick materials (Kulecki, 2002). Moreover, the total cutting cost is relatively high, in large part due to the expenses related to the abrasive material (Pi, 2008).
Today, both AWJs PWJs are widely used in the industry for a large number of applications: PWJ is in used for cutting softer materials such as rubber, leather, fabrics, paper, wood and some kind of plastics (Kulecki, 2002). The technology is also particularly suitable for the food industry, since the absence of knives prevents the transmission of bacteria and other contaminants from one portion to another (Calabrese, 2011). Another industry that can take significant advantage of the cold-cutting characteristics of PWJ is the explosive industry (Borkowski et al., 2008).
Given the ability to cut through harder materials, AWJ is commonly used for cutting metals, stone, glass, ceramic and composite materials, in particular when these are particularly hard to machine with traditional techniques, such as titanium and Inconel alloys (Kulecki, 2002), but also fibrous materials like Kevlar, that cannot be machined by conventional techniques because of pullouts of the fibers (Khan and Yeakub, 2011). AWJ can also be used in combination with traditional machines, as example oxy-fuel cutting (for deburring) or EDM machining (to predrill holes, as in Kulecki, 2002) and for decorative and artistic applications (Carrino et al., 2001). AWJ also presents several outdoor applications that are not related to the machining of parts: some examples are the dismantling of offshore or military structures and nuclear power plants (Louis et al., 2007), the drilling of hard rock in the mining industry (Lu et al., 2013) and the grinding of tires in the recycling industry (Holka et al., 2013). Finally, a last example that well illustrates the capability and the potential of the AWJ technology is represented by its applications in medicine, using a sterile solution as cutting fluid and soluble substances (such as sugar) as abrasive to perform surgeries (Hreha et al., 2010).
DESCRIPTION OF AN AWJ MACHINE
A waterjet cutting machine can be divided in several different sub-systems: as schematized in figure 1, a water preparation system, a pressure generation system and a jet former are required to produce the high pressure jet used for cutting the material (Pi, 2008).
The water preparation system is used to pre-treat the inlet water in order to meet the quality requirements and reduce wear of the components in both the pressure generation system and the jet former. Suspended solids in the water can impact the orifice, causing premature wear, while dissolved solids can precipitate out of solution. On the other hand, water that is too pure also causes problems, since it tends to dissolve the materials it comes in contact with (xinology.com, 2014). Water should therefore be treated with appropriate equipment: softening, deionization or reverse osmosis are all processes that can be used to pre-treat the water according to the quality of the original supply (WARDJet Inc., 2014a). Another factor that must be taken into consideration is the water temperature, since the water warms up when it is pressurized. In warm locations, this may represent a problem, and a chiller system is required to reduce the temperature (to approximately 20 °C) and thus slow down pump’s wear (xinology.com, 2014). Finally, a last task that may be carried out in the water preparation system is the addition of polymeric additives to the water to improve the jet characteristics. Although promising, this last technique is not widely adopted for AWJ machining, since concerns exist about the mixing of the abrasive in the water jet (Kulecki, 2002; Louis et al., 2003);
The pressure generation system includes an electric motor with an output power that lies in the range between 20 kW to 200 kW driving a pump that increase the pressure in a separate hydraulic system. This pressure moves a cylinder (named intensifier) that raises the water pressure up to the required level, usually in the range between 300 and 600 MPa (Lorincz, 2009). In some intensifier designs, the alternate motion of the cylinder causes excessive pressure fluctuations, that must be dampened through a tank with approximately 2 liters of capacity, marked “attenuator” in figure 1 (Arleo, 2010); alternative pump designs, with direct drive and no need for attenuators are also possible (Pi, 2008);
In the jet former (also named cutting head) the high-pressure water is forced through an orifice, producing a high-speed jet of water. At this point, the abrasive material is introduced, together with some air, inside the mixing chamber, where the abrasive particles are accelerated by the high-pressure water before the water-abrasive jet enters the focusing tube (or nozzle) to be directed towards the workpiece. The cutting head include several parts that are subject to wear and tear, such as orifice and focusing tube, and they must therefore be replaced at intervals that depend on various factors, such as water quality, accuracy requirements and materials used: orifices can be made of sapphire, ruby or diamond, with lifetime varying consistently between the different materials, while nozzles are usually made from carbides and can be affected, among other factors, by type of abrasive used.
For machining operations, the cutting head is mounted on a CNC system that allows to control in details all the different parameters. With a properly dimensioned system, it is possible to mount two or more cutting heads powered by the same pump on the same CNC machine, usually achieving better economies (Pi, 2008).
Two other subsystem must be mentioned, in addition to those indicated in figure 1: the catcher and the abrasive supply system. The catcher is a container that has the purpose of collecting the water and abrasive jet after it passes through the workpiece: since the jet still retains much of its initial energy, the catcher must contain some kind of absorbent material in order to slow down the jet without damage. Typically, water with a depth of at least 60 cm is used as absorbent material, but other catcher designs have been developed for mobile applications (Kulecki, 2002).
Finally, the abrasive supply system is designed to provide the right amount of abrasive in the mixing chamber and it is usually computer-controlled to provide an optimal value of the abrasive mass flow (Henning and Westkämper, 2006; Zeng and Munoz, 1994); Holmqvist and Honsberg (2007) developed a model which ties the optimal abrasive mass flow to the water flow through the abrasive/water ratio, which usually lies around 0,2.
Typical abrasive mass flow rates range from 0,1 kg/min to 1 kg/min (waterjets.org, 2014). Water consumption depends on the pressure and diameter of the orifice, and typical consumption ranges from 1,9 liters/min to 4,7 liters/minute (xinology.com, 2014). These values for the water consumption are comparable to the ones of a common household tap and well explain why it is not usually considered to be a problem in the waterjet industry.
From a cost perspective, garnet is by far the largest component of the operative cost, with a share that different studies set in the range between 50% and 75% (Babu and Chetty, 2006; Hashish, 1983; Hoogstrate et al., 2006; Kurd, 2004). Nozzle and orifice costs account respectively for approximately 3% and 1%, while water cost is only responsible for 1,3% of the total (Hoogstrate et al., 2006);
As mentioned above, the abrasive material plays a key role in the AWJ cutting processes, significantly influencing its cost, performances and reliability. This is not surprising, since the whole cutting power of an AWJ derives entirely from the cumulative effect of the impact of a large number of abrasive particles on the workpiece. This process is known as abrasive erosion and has been examined in a large number of studies in an attempt to obtain a reliable model that could be applied to AWJ cutting: most of these models also include some elements regarding the characteristics of the abrasive particles (see as example Hashish, 1987, 1984) or the more recent Gent et al., 2012);
AWJ cutting performances vary with the material, and different and optimized erosion models have been developed for different types of material, such as metals (Hashish, 1984), ceramics (Zeng and Kim, 1996) or composites (Wang and Guo, 2002). Despite the large amount of research, the abrasive erosion process is still not completely understood, and while the studies contributed to an accurately describe the effect on the cut of machine parameters (such as traverse rate and pressure) it is still unclear which is the effect of the abrasive characteristics such as hardness, shape or particle size. Generally speaking, the harder the material that needs to be cut, the slower the speed and the larger the abrasive consumption, while coarser abrasive allows to achieve faster cuts, but increase the surface roughness (Gent et al., 2012).
However, the lack of a comprehensive scientific model describing the influence on the cut of the physical characteristics of the abrasive particles has not prevented the AWJ industry from building up a list of abrasive qualities that are commonly requested to a good abrasive material (waterjets.org, 2014):
• Hardness is often considered the most important requirement of an abrasive material, since the depth of cut increases with it. On the other hand, this increase can only be observed up to a certain value of the ratio between abrasive and workpiece material hardness (Hashish, 1983);
• Sharpness of the particles is also considered an important characteristics, since particles with sharp edges offer better performances compared to more rounded particles (Gent et al., 2012; Hashish, 1983);
• High density (specific weight) is considered an advantage, since heavier abrasive particles can hit the workpiece with more energy (Gent et al., 2012);
• Consistent particle size of the abrasive implies that most of the larger and smaller particles have been removed. This is a desirable quality, since both can cause problem such as nozzle plugging and inefficient cutting (Ohman, 1993); Moreover, the particle size distribution is able to influence the cutting performances (Momber and Kovacevic, 2000).
• Purity of the abrasive is another desirable quality, since softer or harder particles in the abrasive may lead to reduce performances or increased nozzle wear (Ohman, 1993);
• Price is another fundamental factor that must be considered in the choice of abrasive, being responsible for a percentage that varies between 50% and 75% of the total operative costs (Babu and Chetty, 2006; Hashish, 1983; Hoogstrate et al., 2006);
• Low free silica content in the abrasive is finally another advantageous characteristics of waterjet abrasive, since inhalation of silica particles may lead to silicosis, a serious lung disease. Even though the waterjet cutting process in itself produces little or no dust given its wet characteristics, additional precautionary measures should still be taken during the life cycle of the abrasive if this contains high levels of free silica.
ABRASIVE MARKET AND GARNET PRODUCTION
Even though a large number of materials presents all the aforementioned characteristics, the abrasive market seems to be dominated by a single, specific abrasive: almandine garnet, a naturally inert mineral, with a hardness of between 7,5 and 8,5 on the Mohs scale and a density of 3,9 to 4,1 g/cm3.
Almandine garnet generally occurs in schists and gneisses or in placer deposits derived from these rocks. The latter are the source of the majority of the world’s production, since garnet occurs as sand or gravel that with a concentration of approximately 30% and can be easily mined (NSW Department of Primary Industries, 2007; Olson, 2000) either alone or together with other minerals such as ilmenite, rutile and zircon (VV Mineral, 2014) . Extracting the garnet through blasting and crushing the rock is also possible and usually this method produces a higher quality abrasive, since the particles have sharper edges compared to the alluvial garnet which has been subject to erosion. After the extraction, the garnet is sifted and divided into different particle sizes. For waterjet cutting applications, these range from 425 µm (for faster cuts with reduced precision) to 125 µm (for minimum tolerances), with most applications carried out with an average particle size of approximately 200 µm.
A survey carried out in 1994 among a sample of waterjet users (with a strong prevalence of North American users) showed that 90% of the survey respondents used garnet at least some of the time, with most of the high volume users using exclusively garnet (Mort, 1995); the large adoption of this abrasive material has been also confirmed for the Scandinavian market in recent times (Bengtsson, 2014).
Even though garnet is a common mineral, large economically viable deposits of high-quality garnet are relatively rare, and the production is concentrated in a limited number of countries: in 2013, approximately 1.700.000 tons of raw garnet were mined worldwide, with a sharp increase since 2010, when garnet production was estimated in 291.000 tons (Olson, 2002) . The growth is in large share due to the manifold increase of the production in China and India, as shown in table 1:
India is the single largest garnet producer, with a production of approximately 800.000 tons, followed by China (510.000 tons), Australia (260.000 tons) and United States (47.000 tons). Other countries (like Russia and Turkey) only contributed for 83.000 tons, mainly destined to internal use (Olson, 2014).
Only 55% of this amount is considered to be suitable for the market after the refining process, with AWJ industry being the single most important end user in the United State, accounting for approximately 35% of the refined garnet. Other uses include blasting (30%), water filtration (20%) and abrasive powders (10%) (Olson, 2014).
Table 1 also shows how garnet production is highly concentrated in a handful of countries, with little or no garnet produced in Europe. As a consequence, waterjet operations must rely entirely on garnet supplies originating in India, Australia, China or the United States, with transport and logistic operations contributing for a large share to the cost and the environmental impact of waterjet cutting.
The Scandinavian market (including Denmark, Finland, Norway and Sweden) imported approximately 16.000 tons of garnet for different uses, mainly from India and United States. Sweden alone imported approximately 8000 tons of garnet (Bengtsson, 2014).
As mentioned above, even though almandine garnet seems to be the preferred choice when it comes to waterjet abrasives, a certain number of other materials present most of the characteristics listed in section 2.3. In the survey realized by Mort (1995) a detailed overview of the abrasives that are most used in the industry is given. As reported in table 2, garnet is by far the most used material, followed by olivine, slag, aluminum oxide and silica sand. Other synthetic abrasives (such as crushed glass) also played an important role.
Table of contents :
2 Material flows in AWJ industry
2.1 Origin and applications of AWJ
2.2 Description of an AWJ machine
2.3 Abrasive characteristics
2.4 Abrasive market and garnet production
2.4.1 Alternative abrasives
2.5 Management of the waste flows
2.5.1 Alternative recycling and disposal practices
3 Theoretical framework
3.1 Industrial ecology
3.2 Environmental system analysis
3.3 Life Cycle Assessment
4 Research design and methods
4.1 Scope definition
4.1.1 Scenarios development
4.2 Inventory analysis
4.2.1 Reference scenario: Alluvial abrasive and landfilling
4.2.2 Production scenario 1: Crushed rock abrasive
4.2.3 Production scenario 2: Recycled glass abrasive
4.2.4 Production scenario 3: Synthetic high-performances abrasive
4.2.5 Disposal scenario 1: In-site recycling
4.2.6 Disposal scenario 2: Off-site recycling
4.2.7 Disposal scenario 3: Recycling as construction material
4.3 Impact assessment
5.1 Analysis of the scenarios
5.1.1 Reference scenario: Alluvial abrasive and landfilling
5.1.2 Production scenario 1: Crushed rock abrasive
5.1.3 Production scenario 2: Recycled glass abrasive
5.1.4 Production scenario 3: Synthetic high-performances abrasive
5.1.5 Disposal scenario 1: In-site recycling
5.1.6 Disposal scenario 2: Off-site recycling
5.1.7 Disposal scenario 3: Recycling as construction material
5.2 Comparison of the scenarios
5.3 Validity of the results
6.1 Implications for the AWJ industry