Recycling Abrasives

November 1, 2005
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Hydrocyclones can provide an efficient way to recycle used abrasives and reduce the cost of grinding and polishing operations.

It is estimated that almost $5 billion per year is spent on abrasive materials of all sorts in the U.S.1 In addition to the cost of the abrasives, companies that use these materials in grinding and polishing operations must dispose of them when the abrasives no longer achieve the desired process results, and disposal can be costly. As companies look for ways to reduce manufacturing costs, the idea of recycling used abrasive material back into the process becomes increasingly attractive. But is there a way to do so efficiently?

Figure 1. Representative particle size distributions of fresh (blue) and used (red) abrasive materials. (The volume % is arbitrary.)
As shown in Figure 1, an abrasive material will have a wider particle size distribution after use in a grinding/polishing process. The extra-fine particles are primarily abrasive materials that have been reduced in size due to contact with the work surface. Material from the work surface in the form of both fine and coarse particles also contributes to the particle size distribution. Since these out-of-specification particles significantly change the properties of the abrasive material, they must be removed through some form of classification if the abrasive is to be reused.

One approach might be to use screens, which can be arranged so that a single pass would, in theory, remove both the coarse and fine size fractions. However, the screens would need to have a large cross-sectional area and would require mechanical energy to keep the gaps clear so that smaller particles could pass through. Screens also tend to have clogging problems that could result in a bottleneck for some of the in-specification particles.2

Figure 2. Schematics of two types of hydrocyclones. A standard configuration is shown on the left, while a double-overflow configuration is pictured on the right.

The Hydrocyclone Classification Principle

An alternative classification device for recycling abrasive material is a hydrocyclone. Like screens, hydrocyclones have a simple design and are therefore relatively inexpensive. However, hydrocyclones do not require added mechanical energy, which makes them more efficient than screens. They also have no moving parts, which minimizes maintenance requirements.

Hydrocyclones classify particles based on the application of centrifugal force and typically consist of a cylinder on top of a funnel. The most standard arrangement features single orifices in the top of the cylinder and the bottom of the funnel. However, some hydrocyclones have two concentric orifices in the cylinder, and these are generally more efficient at removing coarse particles. The two types of hydrocyclones are shown in Figure 2.

The material to be classified is introduced into the cylinder tangentially under pressure. Particles travel from the cylinder into the funnel by rotating down the surface. As the particles rotate down the funnel, centrifugal forces produced from the moving material separate the particles based on size. The larger particles gravitate to the funnel surface, while the small particles move toward the center. A countercurrent carries these smaller particles back into the cylinder and out the orifice(s) at the top, while the large particles continue down the funnel and exit the orifice at the bottom.

The efficiency with which a hydrocyclone separates at a certain size depends on several design parameters, including the length of the funnel, the width of the cylinder and the size of the upper orifice, as well as on operating parameters such as the flow rate of the input material. As a general rule, the smaller the width of the hydrocyclone, the better it is at separating smaller particles. Given the mean particle size (5-30 microns) and range of many commonly used abrasive materials, a typical hydrocyclone used in this application might have a width of 10-30 mm. In applications where a high throughput is needed, multiple hydrocyclones can be used in a parallel configuration.

Figure 3. a.) Volume-weighted particle size distributions of source (blue) and overflow (yellow).

Hydrocyclones in Action

To examine how a hydrocyclone can be applied to recycling abrasive material, consider the volume-weighted particle size distributions of the alumina abrasive material shown in Figure 3. The goal of this classification was to remove the coarse particles that resulted from the use of the abrasive in a polishing process. The target fraction was the material that came out of the top of the hydrocyclone. (This fraction is usually referred to as the overflow, while the fraction that leaves the bottom of the funnel is called the underflow.)

A single hydrocyclone with a width of 9 mm and a funnel length of 79 mm was used for this classification. The hydrocyclone featured two concentric outputs in the cylinder section, so the actual target fraction was obtained from the innermost output. The so-called middle fraction was returned to the source material and passed back through the hydrocyclone to reduce the number of large out-of-specification particles that would end up in the overflow. The overflow orifice was 1.5 mm, and the underflow orifice was 1.3 mm.

Figure 3. b.) Absolute volume-weighted particle size distributions of source (blue) and overflow (yellow). Distributions were measured using the SPOS technique.
Almost 30 liters of a 3 wt. % dispersion were passed through the hydrocyclone at a flow rate of 1 liter per minute. The blue curve in Figure 3a represents the volume-weighted particle size distribution (PSD) of the source material (the used abrasive), while the yellow curve represents the PSD for the overflow. The goal was to remove as much of the material greater than 5 microns as possible, and then reintroduce the remaining material into the polishing process.

As shown in Figure 3a, a significant amount of source material comprised particles greater than 5 microns-about 87% by volume, as determined by the single particle optical sizing (SPOS) method.3 After classification, the volume of material greater than 5 microns was 50% by volume. The volume-weighted mean diameter dropped from 28.5 microns to 15.8 microns, and the D90 (the diameter under which 90% of the volume is contributed) dropped from 53.2 microns to 39.4 microns.

However, by using the capability of the SPOS method to measure particle concentrations, the real effect of the hydrocyclone can be seen. Having this quantitative information allows the source and overflow distributions to be compared on an absolute volume basis. The PSDs normalized to absolute volume are shown in Figure 3b. In this case, the hydrocyclone removed almost all of the material greater than 5 microns. The little bit of volume left amounts to 9.6% of the original material. This means that the hydrocyclone was slightly better than 90% efficient in this application.

Remember that this was achieved with a single hydrocyclone. The size of the orifices (1.3 mm being the smallest) ensured that that the device would not clog. Furthermore, given the size of the particles and the desired cut-off diameter, such a classification could not have been performed by screens.

It is also important to note that this removal rate was achieved by a hydrocyclone with two overflows. As mentioned previously, having an extra overflow that is channeled back to the source increases the efficiency of coarse particle removal, especially when recycling abrasive materials.

Figure 4. Absolute volume-weighted cumulative particle size distributions of source (blue), overflow from standard hydrocyclone (red) and overflow from a double-overflow hydrocyclone (yellow). Distributions were measured using the SPOS technique.
To demonstrate this greater removal efficiency, another classification was performed on the same used alumina abrasive material, this time using a hydrocyclone with only one overflow orifice. The overall dimensions of the hydrocyclone and its operational parameters were similar to the first classification. The results of the SPOS analysis are shown in Figure 4. These cumulative volume-weighted PSDs represent the abrasive material before and after classification, but with the results of the overflow from Figure 3 included for comparison purposes. The PSDs were normalized to absolute volume.

As can be seen in Figure 4, the overflow from the standard hydrocyclone contained considerably more material greater than 5 microns than was observed in the overflow of the first classification. The mean diameter and D90 measurements of the second overflow were 27.3 microns and 48.9 microns, respectively. These values are only slightly smaller than those of the source material. In making use of the concentration information provided by SPOS, it was determined that the second hydrocyclone removed only 58% of the material greater than 5 microns, compared to 90% for the double-overflow hydrocyclone. This represents a significant drop in removal efficiency and demonstrates that the double-overflow type performs better when recycling abrasive material.

Reduce, Reuse, Recycle

These days, a great deal of emphasis is placed on recycling and waste reduction in all industries. More and more paper mills have been refitted to recycle all forms of used paper products; coating manufacturers are offering their users aqueous-based formulations as a means of reducing organic solvent waste; and even the pharmaceutical industry, through the Process Analytical Technology (PAT) initiative, is trying to reduce waste. It makes sense, then, for users of abrasive materials in the ceramic and glass industries not to treat abrasives as straight disposables. Besides the cost of the initial product, which is relatively high, users of abrasives must also consider the cost of disposal.

Companies looking to recycle used abrasive materials would do well to consider hydrocyclones. These inexpensive devices require little maintenance and can be designed for specific materials and particle size requirements, including classifying particles that are too small for screens.

For more information about recycling abrasives with hydrocyclones, contact Particle Sizing Systems, 8203 Kristel Circle, New Port Richey, FL 34668; (727) 846-0866; fax (727) 846-0865; e-mail pohagan@pssnicomp.com; or visit www.pssnicomp.com.

References

1. http://www.freedoniagroup.com/Abrasives.html.

2. O'Hagan, P.; Hasapidis, K.; Nicoli, D.; and Pokrajac, G., "SPOS, A Unique Tool for Sizing Aggregates," 1999 Fine Powder Processing International Conference Proceedings, State College, Pa., 1999, pp. 78-86.

3. O'Hagan; P.; Hasapidis, K.; Helsing, H.; and Pokrajac, G., "Shedding New Light on Grinding and Polishing," Ceramic Industry, Vol. 154, No. 11, 2004, pp. 14-16.

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