The Classification Of Micron Diamond

Micron diamonds are too small to be sized by sieving (it can plug the hole in no time). Hence, fluid separation is used to separate them. The fluid used can be either water or air. Typically, sedimentation is performed in water, and in certain cases (e.g. for separating submicron particles) centripetal force may be applied to accelerate the settling.

The sedimentation method is based on Stokes' Law for particle settling in any fluid. If the particle is near spherical in shape, its terminal velocity may be expressed as follows:

18n where Vt is the particle's terminal velocity, g is the acceleration of gravity or centripetal force, pd is diamond's density (i.e. 3.52gm/cc), pw is the density of the fluid (i.e. 1 gm/cc for water), n is the viscosity of the fluid (i.e. 0.01 gm/sec-cm for water), and d is the diameter of diamond particle.

For diamond particles settling in water, the following equation may be used to approximate the terminal velocity:

Based on this formula, the settling distance and time for diamond particles of various sizes can be computed as shown in Table 5.1.

For a typical settling distance of 20 cm, the diamond fine with size of 1 ¡m may take 40 h that is extremely slow. By putting water

Table 5.1. The settling distance and time of diamond particles.

Size Velocity Settling Distance Settling Time/cm

40 0.22 787 4 sec

20 0.055 197 18 sec

10 0.014 49 1 min

5 0.0034 12 5 min

0.5 0.000051 0.1 6 h in a centripetal machine, the above settling time may be reduced to only minutes.

Stokes' Law is applicable only to spherical particles. As diamond particles are not spherical, so their settling time are longer than that predicted Eq. (5.2). In the case of irregular shaped diamond, the increase of settling time may be excessive. Hence, large irregular shaped diamond may settle at the similar speed as small round ones. Sometimes the size difference can be as large as 3 times. Hence, the more irregular the shape, the broader the diamond size distribution. The size distribution is critical for precision polishing, so more blocky shapes are preferred. In this way, the chance to cause scratch to work piece is less because less sharp protrusion is associated with blocky shape.

A typical procedure for sedimentation is described as follows:

(1) Mixing upgraded diamond fines with deionized water to form a homogeneous suspension of diamond fines.

(2) Pour the liquid in a sedimentation tank to allow diamond fines to settle.

(3) After a certain time, siphon out water from a fixed depth. In this case all particles still suspended are smaller than a predetermined size.

(4) The water in the bottom of the tank contains mostly larger particles, but there are also small particles that began to settle from close to the bottom.

(5) Fill deionized water in the tank to reach the same water level and thoroughly stir the settled diamond fine to make them suspended again.

(6) Repeat the same process several times to separate the smaller diamond fines over and over until the solution contains almost all large particles.

(7) Combine the solution with smaller diamond fines and repeat the sedimentation to separate even smaller particles.

The above sedimentation process can be processed by dedicated machines with computer control.

During the sedimentation process, in order to allow free settling of diamond fines, the concentration of particles must be limited. Based on experience, if the volume content of the diamond fines is less than 1%, the interference of settling is minimal. Hence, the threshold level of concentration may be determined to be:

Diamond weight/water volume = 0.01

Based on the above concentration and assuming that the distance for diamond fines to settle is 20 cm, then the amount of processed diamond in each batch is about 2.2 R2g, where R is the radius of the water tank in centimeters. In most cases, if the grading of diamond fines is not critical, the above diamond fines concentration and also the batch size may be increased by 7 times. Hence, for a settling tank with a diameter of 50 cm, about 10 kg diamond fines may be treated each time. But because the settling time is rather long, only 15 batches may be processed each year with a throughput of about 150 kg per tank.

During the sedimentation, diamond fines may agglomerate due to the accumulation of electrical charges. In this case, the enlarged diamond clusters may settle much faster and they can mix in with smaller particles. In order to prevent this from happening, a proper sulfactant may be added to the solution. The sulfactant contains cations that may surround diamond fines that are often charged with electrons. In this case, the repulsion of sulfactant cations can keep diamond particles separated so they may settle more freely (Fig. 5.4).

The common sulfactants include sodium silicate, Arabian balsam, sodium phosphate, olive oil, oxalate acid, etc. One example is to add about 0.04 wt.% of the sulfactant in solution.

After sedimentation, the graded diamond fines are fully cleaned. For special applications, such as that for making electroplated tools, diamond surface must free of any metal contamination (e.g. iron from the catalyst inclusion) lest nickel will

Agglomerated Dispersed

Figure 5.4. The agglomerated micron diamonds (left) and their dispersed individual particles (right).

Agglomerated Dispersed

Figure 5.4. The agglomerated micron diamonds (left) and their dispersed individual particles (right).

Figure 5.5. The size distribution of well-graded diamond fines produced by Tomei.

be deposited following the electrical conducting path. In this case, diamond's surface can be etched by oxidants such as by molten salts (e.g. KNO3). The etched diamond must again be thoroughly rinsed in deionized water to remove any salt contamination.

The graded diamond fines must be characterized for their size distribution and particle shape. The characterization may be done by using laser projection instrument, scanning electron microscopy, optical electrical measurement, etc. For example, the cross section of diamond particles can be projected and magnified by 3000 times to achieve a resolution of 0.1 fim (Fig. 5.5).

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