3. S TATE OF THE ART IN ABRASIVE RECYCLING
3.1 - Introduction
In the cutting operation with abrasive water jets a big amount of process waste is produced, compared to conventional machining processes. The process waste is a water-based suspension with abrasive particles and metal from the treated workpiece. The size of the abrasive particles is limited to a narrow range for an optimal performance in the cutting cell and the upper limit of the particle diameter is depending on the cutting head. Cutting heads used in industry have a focus nozzle diameter of 1 mm, which results in a maximum particle diameter of 500µm. The lower limit of the particle diameter is determined by the need to keep to an high level the cutting performance of the abrasives: this leads to a minimum particle diameter of 80µm [25]. Industrial abrasive water jet cutting machines work with an abrasive mass flow rate from 0.5kg/min to 2 kg/min and, based on a daily work period of 7 hours, this leads to an abrasive consumption of 210 to 840 kg/d. This indicates the recycling potential of this manufacturing process and the interest for economical and environmental reasons. Today, the most commonly system used to introduce the abrasives intro the high speed water jet is the injection principle, already explained in Chapter 2. This system requires dry, floatable abrasives to avoid blockages in the feeding pipe or in the mixing chamber; the mixing process of the solid and liquid phase itself takes place at the entrance section of the abrasive nozzle and directly affects the fragmentation of the particles. The first investigations on the reuse of abrasives have been done in the cutting process using the injection principle: Guo et al.[25] found that the cutting performance is dependent on the abrasive particle size and shows a maximum between a particle size of 125 and 200µm. They could recover the 68% of the used abrasive in their cutting process based on a minimal particle diameter of 90µm. With proper cleaning and sorting, an important portion of process waste may be recycled as abrasive material and fed back to the cutting process. Only the remaining portion, the microchips of the workpiece material and the finer abrasive particles have to be disposed. The most important aspect of any recycling system is the method used to separate and classify the dirty abrasive: experience has shown that systems which utilize a rotary drum (or screen) followed by a gravity fed air-wash are very effective for recycling garnet. A vibratory screen below the air-wash has proven itself useful for removing additional contaminants
and improving the level of cleanliness of the recycled garnet. Other major components of a typical recycling system include an inlet hopper, bucket elevator, integrated storage hopper (to store recycled abrasive) and a dust collector. Depending on the application, a level of cleanliness of 90-95% should be possible. Several manufacturers produce recycling systems that are durable and effective at recycling garnet abrasives: a recycling unit designed specifically for a ferrous abrasive such as steel grit may not be effective at recycling garnet. This is due to the difference between the specific gravity of garnet and the specific gravity of steel grit. For a field recycling operation, a collection tank (for storing the abrasive to be recycled), a fresh abrasive storage hopper and a waste bin/drum are also needed. Ensuring that the recharged abrasive (known as the "working mix") is clean and contains abrasive particles with the appropriate size range involves two basic operations:
the removal of oversized and undersized contaminants and the addition of fresh (make-up) abrasive. In order to accomplish this efficiently, both the used and fresh garnet must be kept dry. The most critical element of the recycling process is the creation of an effective
"working mix": given that a portion of the garnet breakdowns in the cutting, a productive working mix should be created by adding fresh abrasive to the cleaned abrasive at a rate equal to the fragmentation. These additions should be made at regular intervals and be based on the amount of fines extracted by the separation unit, daily visual inspections of the recycled abrasive and periodic sieve analyses. It is also critical to remove as many fines as possible during each recycle. Excess fines can reduce productivity, create unwanted dust during cutting and clog the nozzle. The recycled abrasive should be checked to ensure that the separation unit is removing both oversized coarse contaminants (typically larger than 500µm) and the fines (typically 80-90µm or smaller). The most accurate way to determine the exact distribution of the working mix of the abrasive particles is to conduct a sieve analysis. Complete instructions and procedures on the general use of the test sieves are contained in ASTM STP 447, Manual on Test Sieving Methods. For a detailed sieve analysis, use #20, #30, #40, #50, #60, #70, #80, #90, #100 and #120 mesh USA Standard Sieve screens. A short-cut method used to reduce the complexity of conducting a complete sieve analysis is to select the "critical" screen for the application where recycled garnet is being used. This will give a relative indication of how well the recycling process is being managed and point out any problem. Routine visual checks should be made by qualified personnel at the discharge tubes leading from the air-wash separator and rotary drum at least 2-3 times per shift. Check for usable abrasive in the discharge and fines in the recycled material. Check for a uniform and full abrasive curtain in the air-wash. If not cleaned out regularly, dust builds up as abrasive stacks up before the swinging baffle. This causes gaps in the abrasive curtain and negatively effects the efficiency of the air-wash.
The commercial options available to accomplish garnet recycling have high excess capacity, elevated water and energy usage and a high capital cost. The new systems that are being developed should provide operational flexibility and should include sedimentation of particles, magnetic separation (to recover ferrous contaminants), screening (to meet size specifications) and water flotation to separate the more dense abrasive material from the lighter debris. A new requirement is the provision of a water disinfection system to address health and safety issues: a chlorination disinfection system has been incorporated in the system. Moreover, the unit should include several layers of screens, to provide more than one size for reusable garnet, in order to provide the preferred abrasive size for each different application in the company.
3.2 - Research on particle fragmentation
As already told before, the breakdown rate (or fragmentation rate) of the abrasive particles is a very important aspect to take into account when talking about abrasive recycling.
A study from Kretschmer and Aust [26] points out the importance of the particle break up during the acceleration process in the mixing chamber that leads to a change in the size spectrum. After passing the cutting head the abrasive water jet hits the workpiece and the particles are further reduced in size due to the impulse exchange during the abrasion. But the particle break up taking place in the cutting is small compared to that in the mixing process. The most important results of this study is the fact that the recycling capacity, that means the share of abrasive particles which can be reused for cutting operations, remains constant after the second cycle.
Another study conducted by Labus et al. [7] is focused on the effects of various process parameters, on the particle size distribution in an abrasive jet. The characterization is done by collecting abrasive samples before entering in the mixing chamber, after exiting from the mixing tube and after cutting a test specimen. The abrasive used for this study was a
#80 garnet. The main mass fraction of this nominal #80 grit (particles in the range 180÷250µm) was used to provide a more controlled initial size range. The first series of test were done with the abrasive jet issuing into a catcher without impinging on a target and a first result of the analysis of particle distribution after mixing is that the 75÷150µm size range is not influence by parameters changes during transit through the nozzle or during the cutting process. Of the particles in the initial range of 180÷250µm, which produces the majority of the cutting action, only the 20÷22 % survive the passage through the cutting head. Particles in the size 75÷150µm don’t seem to be affected during cutting operation.
The breakdown process during the cutting action may be a shift from the main mass
fraction into the intermediate sizes and finally to the sizes less than 63µm. But this seems improbable because of the consistent repeatability of the mid-range particle size distributions and the matching gain/loss of the fines versus the large size particles. The shift to the fine particle size is most probably a direct breakdown of the larger sizes rather than a general shift from size to size.
For the abrasive size reduction during mixing process several authors measure that about the 70÷80% of all the particles are subjected to fragmentation and find that this number mainly depends on the original abrasive grain size, pump pressure and focus diameter. To quantitatively evaluate the process of abrasive particle disintegration, Ohlsen [27]
introduces a “disintegration number”, defined as (3.1):
1 Pout
D
Pin
d
Φ = − d (3.1)
With 0 < ΦD < 1. If ΦD = 0 no disintegration occurs; typical values for garnet are between 0.15 and 0.70. In the Fig.3.1 is shown the influence of several process parameters on the disintegration number.
Fig.3.1a,b,c,d – Influence of pressure, focus length, focus diameter and mixing chamber design on disintegration number [9]
Fig.3.1e,f – Influence of abrasive mass flow rate and abrasive particle diameter on disintegration number [9]
The Fig.3.1a shows that the disintegration number linearly increases with the pump pressure; the slope decreases at very high pressure. The focus length (Fig.3.1b) has a very weak influence, showing that the particle impacts on the focus wall do not significantly contribute to the fragmentation. More pronounced is the focus diameter influence (Fig.3.1c): as the focus diameter decreases, the disintegration increases from ΦD = 0.22 to 0.46. The mixing chamber design (Fig.3.1d) does not have a major impact on the disintegration number. Fig. 3.1e illustrates that, as the abrasive mass flow rate exceeds a certain value (4 g/s), this parameter does not significantly influence the particle disintegration. Finally, in the Fig.3.1f is shown the influence of the particle diameter: the disintegration number almost linearly increases as the particle diameter increases.
Regarding the fragmentation after cutting, the influence of some process parameters are different: in the Fig. 3.2 can be observed that the effect of pressure is weaker than during mixing; on the contrary, the effect abrasive particle size is linear and the larger particle classes (300÷500µm) fracture more intensely than smaller fractions (150÷300µm). The particle fragmentation is observed to be more intense at high traverse rate and reduced by an increase in the abrasive mass flow rate. The amount of target material that is removed during a given cutting time influences the disintegration process because the single grain performs more destructive work for the removal of a larger quantity of material.
The fragmentation is maximum for steel followed by titanium and aluminium; Labus et al.
[28] noticed an increased abrasive fragmentation for high target material hardness.
The recycling capability strongly depends on the cutting task such as kerfing, rough cutting and quality cutting: in the Fig. 3.2d is shown a linear relationship between the workpiece thickness and the disintegration number.
Fig.3.2 – The effect of cutting parameters on fragmentation after cutting [9]
3.3 - Research on abrasive recycling
The main research on abrasive recycling found in the literature is done by M.K. Babu and O.V.K. Chetty of the Indian Institute of Technology Madras in Chennai. In a paper on abrasive recycling [2] they investigate the recycling of Indian garnet while cutting aluminium using a specially formulated optimised abrasive test sample. The performance of this sample has been compared with that of commercial grade abrasive of #80. This work attempts to study the recycling capabilities (cutting performance and cutting quality) and reusability of local abrasive particles and the recycling studies are undertaken with used abrasives after screening out the particles less than 90µm and also with all particles without screening. The process parameters are constant and are listed in the Tab 3.1 where can be noticed that the water pressure value and the traverse (feed) rate value are quite low for an industrial application of the results of this study.
Tab.3.1 – The constant process parameters in the recycling study from Babu and Chetty [2]
An Aluminium 6061-T6 trapezoidal workpiece has been cut and the maximum possible depth of cut is determined for each abrasive sample. Then three measurements of the surface roughness (Ra) where done on the cut surface and averaged. To study the disintegration behaviour of abrasives, abrasive particles have been collected at the exit of the focusing nozzle and also after cutting and the average particle size has been calculated as explained even in the Chapter 6. The collected abrasive are cleaned from aluminium debris, dried and then sieved. The particles less than 90µm are screened out for improved cutting performance and repeated use but, in order to understand the behaviour of recycled abrasives in the presence of finer particles, studies are also undertaken with all particles for possible reduction in recycling costs.
Recycling with screening (<90µm)
From Fig.3.3 can be observed that the complex process of mixing within the mixing chamber and the focusing nozzle results in a decrease in the aps and that the fragmentation occurring with new abrasives is much more intense than the one with recycled abrasives.
This is due to the fact that fresh abrasives has particles of larger size whose fragmentation is greater. The fragmentation is more pronounced in the test sample than #80 sample because it contains more larger particles. The fragmentation after cutting is much less intense than after focusing and, for the test sample the fragmentation of recycled abrasives after II cut is not significative.
Fig. 3.3 – Effect of recycling of abrasives on aps of test sample (a) and mesh size #80 (b) with particles screening (<90µm)
Regarding the cutting performance in the Fig.3.4 is shown how the test sample performs better than commercial abrasive #80 and both sample perform worse when recycled; hence recycling leads to decreased depth of cut due to the particle size reduction. The decrease is about 20% on the first cut and an additional 3÷5% with further recycling cycles.
Fig.3.4 – Effect of recycling on the maximum depth of cut of samples
The performance of recycled abrasive from the test sample is much more stable with multiple recycling than commercial #80 garnet.
The cutting quality (measured through an averaged surface roughness Ra, Fig.3.5) of the recycled abrasive samples is much better for recycled abrasives that allow to reach values of even 4µm against the 8µm achievable with new samples. The particle size distribution seems to play an important role since the test sample performs better than #80 garnet.
Fig.3.5 – Effect of recycling on surface finish of samples
Recycling with all particles
Due to the disintegration with reuse the abrasive particles become finer and finer and as a result blockage of the flow channel was noticed and the cutting has been erratic. This don’t allow to recycle more than three times. The reduction of maximum achievable depth of cut in this case was from 22 to 26 % after I cut (Fig.3.6); but, what is more important is that in this case the performance with further recycling decrease even for the test sample.
Fig.3.6 Effect of recycling on depth of cut with all particles
However, the depth of cut achieved with removal of finer particles (<90µm) is marginally higher than that of all particles. Regarding the surface finish in this case the #80 sample resulted in decreased performance (greater Ra) with recycling and a fluctuating behaviour that can be seen in Fig.3.7. The test sample performs better with recycling and the performance is more stable than the other sample due to the better beginning particle size distribution.
Fig.3.7 – Effect of recycling on surface roughness with all particles
The reusability with limit for the reusable size of 90µm is shown in Fig 3.8 for both samples. In this study the reusability after I cut (~80%) is very high due to the the lower pressure used in the tests that produce a quite low fragmentation and the fact that the material cut is aluminium. For an industrial application of the results from a recycling studies would seem more appropriate to use a pressure commonly used for metal cutting (360-380 MPa) and to cut a steel workpiece; in fact, the steel causes a greater fragmentation of the abrasive particles. Moreover, the reusability found in this studies is not very significative because done a workpieces with increasing thickness with the consequence of a continuously varying fragmentation; indeed, the fragmentation strongly depends on the cutting task (rough cut, quality cut or kerfing) and this is identified by the ratio of the actual depth of cut and the maximum depth of cut. If the depth of cut is variable the cutting task is variable and the reusability data don’t have a great significance since in the companies the cutting is done with constant thickness workpieces.
Fig.3.8 Reusability for the recycled samples
Another important study has been done still from Babu and Chetty [29] who have investigated the effect of the recharging of Indian garnet abrasives on the cutting performance in order to determine the optimum recharging amount required. The experiment setup and the cutting parameters are the same of the previous study but in this case the abrasive after being collected and sieved has been recharged with fresh abrasives at various proportions respect to the recycled mass. Additional studies are carried out after screening out particles less than 90µm. From the aps (Fig 3.9) of the samples can be observed that a tremendous disintegration occurs with the fresh abrasives in the mixing chamber, compared to the disintegration of recharged abrasive samples. The recharge at 100% means that the proportions recycled-fresh abrasive is 1:1. The more recharged samples have a greater average particle size but after the mixing process the aps of all the samples is practically the same; this agrees with the fact that the fragmentation is more pronounced for greater particle size values.
Fig.3.9 – Effect of recharging on aps
Regarding the maximum depth of cut, with the recharging, it’s expected to increased compared to that without recharging. In the Fig.3.10 is compared the cutting performance of the various samples and can be noticed that, with an increase in percentage of recharging, the depth of cut has increased significantly up to 40% of recharging and marginally after. A level of recharge of 40% seems the most proper for the maximum cutting performance.
Fig. 3.10 – The effect of recharging on depth of cut
The cutting performance with multiple recharging hasn’t been investigated in this study and could be an interesting aspect to deepen. Moreover, this approach of recharge seems not economically effective: a new approach, that will be investigated in this project, is to recharge the abrasive in order to restore the input mass of the sample, after the screening of the smaller particles. The roughness of recharged sample is fluctuating both with Test
sample and with #80 indian garnet. The results with the screening of particles smaller than 90µm are similar to the previous, both for the aps and for the maximum depth of cut.
3.4 - Recycling systems from WARDjet®
The WARD (Waterjet Abrasive Recycling Dispenser) systems are quite famous in field of abrasive recycling and are sold by the Wardjet® company [30]. They are removal and recycling systems that allow to get dry and clean abrasive back from the water tank in three minutes. Indeed, the function of this kind of systems is to remove the sludge from an abrasive waterjet tank catcher, to separate the reusable abrasive from the waste, to wash and dry the reusable abrasive and to deposit it into the feed hopper.
The process begins by removing used abrasive using patented (WARDJet designed) abrasive removal nozzles (Fig 3.11) that are buried on the bottom of the waterjet tank. The scheme of the removing process is showed below in Fig. 3.12. These nozzles have no moving parts, yet are highly effective at being able to move the abrasive out of the tank and to the top of the abrasive recycling machine. A main advantage respect to other systems is that can work even during the cutting and work best when buried under 30 cm of abrasive mass. The nozzles, with the three pumps dedicated, agitate the solid mass to break and transport it to the separation’s device. Here there are a series of vibrating screens where reusable abrasive is separated from waste material.
Fig 3.11: Abrasive Removal Nozzle (Wardjet® company website)
Fig 3.12.: Abrasive Removal system (Wardjet® company website)
After being screened (Fig.3.13) the reusable abrasive is then dried using electric power (Fig.3.14). A very small amount (approximately 1.9÷3.8 l/min) of clean water is used to do a final wash of the abrasive. The bulk of the washing is done with water from the waterjet tank. This water along with any excess water from the WARD overflows back into the abrasive waterjet tank.
Fig 3.13 and 3.14 The drying process and the screening and washing processes performed by the WARD systems (Wardjet® website)
The final results from the whole process are:
• Recycled Abrasive: The recycled abrasive has been washed, dried, and is ready for reuse. An optional splitter will separate the dry abrasive into two grades, increasing the percentage recovery achievable. This abrasive can be used in exactly the same way as the new abrasive and is collected in a storage container (Fig 3.15).
• Waste Material: Sludge, fines and undersize abrasive that is too small to use for cutting are deposited into waste containers to be emptied into a dumpster. This
sludge is typically very compact and has little water in it, since the water used for washing the abrasive overflows back to the tank.
Fig 3.15.: The recycled abrasive discharge (Wardjet® website)
WARDJet® produces two different sizes of abrasive recycling machines. The original WARD 1 is the larger unit (Fig 3.16), which can dry up to about 81 kilograms per hour.
The smaller, more compact, WARD 2 (Fig 3.17) was built to balance a waterjet system using a maximum of about 6800 kilograms per month.
Fig 3.16 and 3.17: The WARD 1 and the WARD 2 recycling systems (Wardjet® website) The WARD 1, with nearly 100 units installed worldwide, has earned a reputation for recycling abrasive reliably, consistently and efficiently. It’s Built for larger users of
abrasive, the WARD 1 is an abrasive removal and recycling system, allowing the automated removal of abrasive from the bottom of the waterjet tank to be delivered directly to the recycling system. The good reusable abrasive is dried and then automatically splitted into two grades of abrasive and delivered up to 18m away. Some of its technical characteristics are listed in the table below (Tab 3.2).
Tab 3.2: Technical characteristics of WARD1 system
The WARD 2 has an integrated method of washing, screening, drying and splitting the abrasive into coarse and fine for reuse. The WARD 2 incorporates a hopper to store the dry abrasive in two grades, a dust and steam collector and a drum to collect all the undersize abrasive and waste kerf material. Its technical characteristics are listed in the Tab 3.3.
Tab 3.3.: Technical characteristics of WARD2 system 3.5 - Recycling system from Jet Edge
The Jet Edge Abrasive Recycling System (Fig.3.18) is designed to remove the sludge from an abrasive water jet cutting tank or integrate into an existing abrasive removal system. The recycled abrasive can be placed in the hopper for use again with the optional abrasive transport system.
Fig 3.18 – The recycling system from Jet Edge
The abrasive is removed from the tank using patented nozzles that have no moving parts.
The nozzles work best when buried under approximately 12" (30 cm) of sludge. The sludge is sent to the top of a series of vibrating screens where usable abrasive larger than 100 mesh is separated. This is then dried and made ready to use again. A very small amount of clean water is used to do the final wash of abrasive, approximately 0.8 gallons (3.0 liters) per minute. The bulk of the washing is done with water from the abrasive tank and is then returned to the tank again. This has the effect of cleaning the water in the tank to some degree. All excess water from the recycling process overflows into the abrasive waterjet tank. The control box has several built-in safety features, as well as override and cut-off defaults. As soon as the abrasive removal nozzle is activated, the recycling process will begin. The Abrasive Recycling System enables operators to reuse the abrasive, cut operating costs and increase profit margins. The product specifications, used even for the economical analysis in Chapter 4 are the following (Fig.3.19)
Fig. 3.19 – The technical characteristics available from website of Jet Edge[31]
