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A Comminution Device for X-Ray Spectrochemical Analysis

Reprinted from American Laboratory, Vol. 25, No. 12, August, 1993

By Dr. Monte J. Solazzi

Sample Substances ranging from single to multiple-compound structures, classified as compositionally complex in this article, comprise combinations of different particles. Each is characterized by its separate chemical and physical properties, e.g., chemical composition, particle size, shape, hardness, and density. In practice, this is a fair representation of the samples generally presented to the analyst for direct X-ray spectrochemical analysis, and as such, they are not acceptable. Any differences or variation in the constituent sample particles with respect to chemical and physical properties are potential factors affecting the X-ray data and degree of analytical accuracy. Comminution reduces these discrepancies to minimal levels.

The objective of the comminution procedure is to reduce the constituent particles to a uniform size, shape, distribution, and level of insignificant influence on analytical data in a statistically reproducible manner. The comminution equipment must be capable of utilizing a single set of operating conditions common in meeting this objective, regardless of initial differences in sample composition complexities and physical characteristics. This imparts a high degree of credibility to the sample preparation procedure with the foreknowledge that each type of sample substance is effectively comminuted. The comminution equipment must also be relatively fast and simple to operate, maintain pace with current and future laboratory needs, and be ergonomically designed to minimize operator discomfort, especially for routine sample processing. The comminution equipment must also provide a means to minimize the introduction of transition wear-element contamination to the sample substance and wear to the grinding vessel and media. The GyralGrinder® (Chemplex® Industries, Inc.), a comminution device featuring variable frequency and intensity of impact control, meets these requirements.

Principle of Operation

Figure 1. Comminution Vessel

The GyralGrinder is an electromechanical device that imparts an eccentric gyral motion to a comminution vessel containing a sample substance and freely mobile grinding media (Figure 1). The eccentric gyral motion is created by inertia of the mechanics of the system to generate a controlled imbalance condition resulting in an energetic motivation of the grinding vessel. Comminution is effected by collision and milling actions by the grinding media on the sample substance.

Figure 2 illustrates a grinding vessel located a distance (d) from a focal point (F) relative to the center axis of the system (C). At rest (Figure 2a), the comminution vessel together with its grinding media and sample are stationary (the grinding media are shown to be in center alignment with the center axis of the system for illustrative purposes). As a moment is initiated by the mechanics of the system pivoting at the focal point F, Figure 2b, the grinding vessel is laterally displaced a distance (d,) from the center vertical axis (C); the grinding media and sample generally lag behind the grinding vessel displacement direction as they are propelled by its trailing side. Subsequent increases in instability, Figure 2c, gyraIly enlarge the displacement distance (d2) of the grinding vessel relative to the' center vertical axis up to a physical limitation imposed by other mechanics integrated in the system. At the limitation displacement distance (d]), Figure 2d, of the grinding vessel, the mechanics gyraIly reverse displacement direction accompanied by a corresponding decrease in displacement distance 4), Figure 2e, in a circular arc configuration (not shown). The grinding elements, however, continue in the same initial travel path with increased speed and inertia propelled by the trailing side of the grinding vessel to impact its interior walls. The reversal of grinding vessel direction in the oncoming path of the sample substance and grinding media is extremely energetic and is primarily responsible for comminution. The cycle repeats itself in the opposite direction, Figure 2f. Furthermore, the reversal of displacement direction of the grinding vessel in a circular arc adds a milling effect to the sample substance created by the grinding media that acquires a spin attributed to the gyral mechanics.

The mechanics of operation are portrayed in slow motion to illustrate the principles involved in this equipment. In practice, the comminution process is extremely rapid and highly energetic. Additionally, by the introduction of control on the number of displacement distance occurrences within an interval of time, the frequency and intensity of impacts become variable. This feature provides the analyst with the capability of adjusting the comminution process to range from a simple gentle grind to a most vigorous and energetic action.

Most importantly, independent operator control of intensity and frequency of impact, together with other features incorporated in the device reduce the comminution process to the use of a common set of operating parameters applicable to all sample substances regardless of their dissimilarities in composition complexity and physical characterization. Control over the frequency and intensity of impact level ensures that transition wear-element contamination to the sample is significantly minimized; in addition to reduced wear to the comminution vessel and grinding elements.

Experimentation

A series of experiments were conducted relating to particle size weight fractions collected from comminuted sample material processing. Four different arbitrary sample substances were selected principally for their relative dissimilarities in composition complexity and physical characterization.

The GyralGrinder can be operated in several different modes: intensity of impact held constant and processing time varied, processing time held constant and intensity of impact varied, and both processing time and intensity of impact varied. A typical representation of the comminution process and experiments to be performed in this report is shown in Figure 3. This relates the particle size weight fraction of the sample collected corresponding to either processing time, intensity of impact, or both. The degree of curvature appearing at the upper end of the curve represents the range at which no further particle size reduction is realized with additional processing for a specific sample material substance, comminution vessel material, and equipment.

Varied Processing Time

The device was operated without benefit of utilizing the variable intensity of impact control, which, in principle, is similar to the operational performance of traditional comminution equipment. Each sample substance was processed for specified time duration and its respective particle size weight fraction collected as previously described. The results of this test are exhibited graphically in Figure 4. As expected, the fine powdered substance required less processing time in producing the greatest percentage of collected comminuted particles. Sample substances initially presented in chunk physical form dictated the longest processing time in generating similar particle sizes. The notation that all curves are widely separated and do not converge is clearly reflective of the dissimilar behavior of different sample substances when subjected to the same processing conditions. Most importantly, this illustrates that the operating conditions employed for a given classification of sample substances are not presumably translatable to another without implementing operational adjustments.

Varied Intensity of Impact

An experiment was performed that involved holding processing time fixed and varying intensity of impact, as shown in Figure 5. A significant improvement in the comminution, process is indicated by the convergence of the individual curves to a point common to the different sample substances with the exception of the chunky sample material, which still presented difficulty as evidenced by its displacement and degree of departure from convergence with respect to the other curves. Ideally, curves generated by dissimilar sample substances should decrease in their displacement from each other and converge at some common point. A common point of convergence represents the highest degree of effectiveness in the process realized for given sample substances, comminution vessel material, and grinding equipment. It is indicative of applying a single set of operational parameters common to all sample substances within the range of analytical interest.

Varied Processing Time and Intensity of Impact

The last experiment was reflective of the instrument's ability to process sample substances of varied compositional complexity and physical characterization. For this test, both processing time and intensity of impact were varied. A GyralGrinder operated in this manner incorporates the benefits of both varied comminution time and intensity of impact processes into a common set of operating parameters. The results of the test are represented in Figure 6. The convergence of all curves at approximately the same point common to all the sample substances clearly demonstrates the comminution effectiveness of the device. The improvement in the comminution process is directly attributed to the use and application of the intensity and frequency of impact control. As illustrated, this feature provides a common set of operating conditions at which virtually any type of sample substance, regardless of dissimilarities in chemical composition and physical properties, is uniformly processed. The resultant sample substances are similar in particle size and homogeneously distributed for improved statistical precision and analytical accuracy for direct X-ray spectrochemical analysis.

Comminution Vessels

Selection of the most appropriate comminution vessel substance is determined by material hardness, for further maximizing of the comminution process and avoidance of transition wear-element contamination to the sample. The tests in this study were performed in a hardened steel comminution vessel, which will satisfy most laboratory applications, and is supplied as a standard item with the instrument. Typical wear-element contamination from a hardened steel comminution vessel is caused by iron, chromium, silicon, manganese, and carbon. It is reasonably resistant to abrasion and very durable for moderate to high intensity and frequency of impact settings.

For applications requiring a harder grinding vessel substance and avoidance of transition wear element contamination attributed to hardened steel, different and harder vessel substances are optionally available. They include tungsten carbide, alumina ceramic, and zirconia ceramic. Generally, a harder comminution vessel material results in shorter processing times and decreased intensity of impact level settings. Empirical investigations and testing similar to those described in this presentation are generally required prior to actual sample substance processing and with each change in comminution vessel material. This procedure will assist in determining the optimum common operating parameters to accommodate the expected types or classifications of samples submitted for X-ray spectrochemical analysis. Once these common conditions are empirically determined and established, they should remain reasonably constant for all subsequent similar sample material processing.

Instrument Features

The GyralGrinder is a freestanding unit at an average determined height intentionally selected to reduce operator bending and fatigue. The operating controls are conveniently located in the top lid cover, which is supported by dual pressurized gas springs, and lifts upward and back out of the way. This enables close access to the grinding vessel chamber in a standing position. With the lid cover in the closed position, the controls are also within easy reach, are spatially located in groups of function similarities, and are accessible in a standing position.

Comminution Vessel Clamping Mechanism

The comminution vessel clamping mechanism employs a uniquely engineered single-hand cam-operated lever locking design that pivots out of the way. This greatly facilitates removal of the comminution vessel and further provides unobstructed access to the comminution vessel chamber in a standing position. The critical components in the clamping mechanism are fabricated from hardened chrome steel for extended longevity of use. A dust rail is also incorporated within the comminution vessel chamber that serves to collect any residual powdered sample substances, maintains the gyral mechanics relatively dust free, and facilitates cleanup of inadvertent spills.

Controls

All operating controls are located on the exterior of the lid cover. They include a lighted pushbutton Main switch, a lighted pushbutton manual On/Off switch, a lighted push-button Momentary Operation switch, a control for the intensity of impact, and a programmable electronic interval timer in I-min increments. The push-button switches are also illuminated in different color codes for further ease of identification. The Main switch supplies power to the unit. The Manual switch is employed to operate the unit without a fixed, timed processing interval. The Momentary switch engages the unit for as long as this control is held down in the "On" position. The timer is programmable for fixed time durations in processing sample substances for similar time intervals in 1-min increments. The device utilizes solid-state electronics and controls, which are also located in the lid cover.

Miscellaneous Features

The instrument is constructed of heavy gauge steel and is electro statically coated with a durable finish. It incorporates sound-absorbing material to reduce noise generation to acceptable limits, casters for intermediate mobility to the installation site, and skid-resistant leveling legs to account for irregularities in flooring. Safety switches and lid cover locking devices are also incorporated.

Conclusion

By the introduction of operator-controllable variable intensity and frequency of impact to powdered sample material comminution, innumerable types of sample substances of varying chemical compositionally complexity and physical characterization are effectively and similarly comminuted by the utilization of a single common set of operating parameters. Transition wear-element contamination to the sample substance and wear to the comminution vessel and components are significantly minimized by controlling displacement distances and frequency of impact occurrences, within an interval of time and associated intensities. Other features incorporated in the instrument are intended to greatly facilitate the comminution process concurrent with providing ease of operation for the analyst.

GyralGrinder® and Chemplex® are registered trademarks of Chemplex Industries, Inc.

Dr. Monte J. Solazzi is President, Chemplex® Industries, Inc. 2820 SW 42nd Avenue, Palm City, Fl. 34990, USA. Tel: (772) 283-2700. The author thanks Mr. Hector Castaneda for his interest in this project and his time and effort in processing the various sample substances and collection of data.