Grinding

Grinding or comminution is the process of reducing the size of particles. In a broader sense, comminution is used to describe desirable size reduction while attrition is used to describe accidental or unwanted size reduction. An adjunct to grinding is classification or particle size separation.

Methods
In general, grinding is divided into media or medialess grinding or milling. In media milling, balls, pebbles, or other media, such as sand, are used in a stirred mixture along with the sample material to be ground. The collisions of the media with the sample material break the particles into smaller pieces.

Common methods of media milling or grinding include ball milling, bead milling, attritor milling, sand milling, horizontal milling, vertical milling, and vibratory milling. In each case, media, typically larger than the material to be ground, is added to a chamber containing the material to be ground. The entire collection is then stirred, rotated or otherwise agitated. By choosing the correct media material (sand, steel, alumina, or zirconia) and the correct size (1 mm to 10 cm), sample particles can be milled down to average sizes of 0.1 micron very readily. By controlling the time, applied energy, and the size of the grinding media, almost any size particle can be obtained. While media mills may be operated dry without any added liquid, it is commonplace to mill the material in a solvent or water with additional dispersants. Wet milling produces the finest particles and provides a ready-to-use dispersion at the same time.

Medialess milling equipment includes jaw crushers, hammer mills, jet mills, and microfluidizers. Grinding is obtained through the impact of the sample particles on solid surfaces, through particle-particle collisions, or through rapid pressure changes resulting in cavitation. Jaw crushers and hammer mills rely on particle impact on solid surfaces and are used to break large particles or chunks (2 cm to 2 m) into 60  micron to one centimeter size particles. While this process is typically used in the mining industry, it also has utility as a first step in size reduction for many other materials and industries. Hammer milling is the process whereby particles are reduced in size by impact with rapidly moving surfaces. An example of such a device has rapidly rotating hammers that strike particles repeatedly until the particles are reduced to a size that can pass through a nearby screen. Hammer milling is typically done at ambient conditions to produce particles of 30-500 microns; however, for temperature-sensitive or soft materials such as polymers, milling can also be done at cryogenic temperatures with frozen particles. Jet milling is a process whereby the particles are suspended in flowing air streams. The entrained and supersonically moving particles are then directed at a target or at themselves resulting in a fine grind without media or added solvents. Typically, a jet milling process can produce particles with an average size of 1-10 microns very readily. The last medialess milling process is high pressure dispersion milling in which dispersions are pressurized to 10,000-50,000 psi and then the pressure is rapidly released resulting in cavitation and grinding. Typical particle sizes from this process range from 0.05-1 micron.

In any grinding process, the particle size distribution that results is Gaussian and, in most cases, is broader than desired. In those cases, the ground material is post-processed to separate it into the desired particle sizes. The simplest method of particle separation uses screens or sieves. The material is placed on the screen and then the screen is shaken to allow the smaller particles to flow through. The “overs” are the particles that remain on the screen and the “unders” are the particles that pass through the screen. In a continuous process, the particles are continuously added to a screen and the overs continuously removed so as to avoid blinding or plugging the screen. Particles that are smaller than 100 microns are usually difficult to screen at high rates. In these cases, the particles are separated using air classifiers. This equipment operates by applying opposing air flows and centrifugal forces. By balancing the two forces, smaller and larger particles can be separated. Good separation is usually obtainable down to two microns. Depending on the size of the screen or the classifier rates, classification can be as low as one pound per hour to as high as thousands of pounds per hour.

Specialty Applications

Medical Milling
Some medical implants are comprised of milled materials that are then added to other material or directly formed into the implant. AVEKA has designated a room and the equipment for processing medical materials and can hammer mill such materials under ambient or cryogenic temperatures. Following milling, AVEKA can screen the milled material to meet size the specifications of the project.

Wear Debris Generation
The manufacturers of medical implants are required by the FDA to perform wear debris studies. AVEKA has helped some of these companies by creating populations of very small particles from their implants. The particle size required is usually less than 5 microns, and AVEKA uses specialized methods to create these particles. Implant materials from titanium alloys to polycarbonate urethane to high density polyethylene have been milled successfully.

Example: Media Milling Garnet Sand.
In this case, we wanted to understand the grinding process to be able to determine the appropriate endpoint of the process.  The particle size mean is a key parameter in the assessment of a size distribution, other parameters of the distribution can be important for the use of the material.  For instance, if plugging of an orifice by an ink or paste is a critical issue in the application of the milled material, then the particle size mean is one indication of the utility, but the critical parameter is the d100, or the size where 100% of the particles are below that number.  Alternatively, if the application is an ink where clarity of the dispersion is the critical parameter, then the mean distribution will give an indication of this parameter, but a better measure will be the fineness of the particle size distribution.  One way to measure this is the d5, or where the particle where 5% of the particles are less than this number.  A smaller d5 will indicate a finer grind and hence a dispersion with less haze.  Below we have two examples of the size distributions obtained from this material as a function of grinding time.  From this data we have plotted the d5, mean, and the d99 of the distributions as a function of time.  Depending on the desired application such as an ink where fineness of grind is critical or a paste that is being extruded through a small orifice and where any large particles can plug the orifice, the particle size mean may not be the measure of the endpoint of the grind.  In the example below, the grinding endpoint for the mean is 2.5 hours, while the endpoint to minimize large particles is 10 hours of grinding, and the endpoint to maximize the fines end without excess milling is 7.5 hours. Controlling the process and producing the desired result is critical in most milling applications. By understanding the process window, we have been able to provide a consistent product.

Characterization

 

Equipment

Within the AVEKA Group, we have many different models of this process equipment which we use to grind and classify a broad range of materials including polymers, pigments, oxides, inorganics, organics, feed, food, and metals. Rates of grinding can be as low as grams per hour and as high as tons per hour. This is dependent on the size of mill used, the material to be processed, the desired size distribution, and the tightness of the size distribution required.