Mills under the microscope

Manufacturers of products such as polymers, pharmaceuticals and food additives commonly use low-temperature cryogenic grinding to cool bulk materials prior to processing them into micron-sized particles. This is achieved through four commonly used types of milling machines – the hammer mill, the attrition mill, the pin mill, or the turbo mill.

Now, in an attempt to clarify the effectiveness of each of these mills, engineers at Air Products has produced a comparative analysis that shows how efficiently each one of them can cryogenically grind three different synthetic materials that are representative of those commonly found in industry.

All of the four milling systems examined in the study reduce the particle sizes of materials through a process of impaction and attrition. In the oldest of the systems – the hammer mill – material is impacted by hammer bars inside a steel drum which shred it before expelling it through screens in the drum.

Unlike the hammer mill, neither the attrition mill nor the pin mill uses a screen to produce the required particle size. Instead, the attrition mill pulverises the material between two toothed metal disks rotating in opposite directions, while the pin mill uses two opposing, rotating surfaces with tightly spaced pins.

The turbo mill is the most recent development in cryogenic milling. It uses a combination of three size reduction mechanisms, including impact, attrition and particle-particle collisions, and is claimed to be more effective at grinding difficult-to-grind polymers to much smaller particle sizes than current impact mills.

Jon Trembley, lead engineer at Air Products’ European Centre of Excellence of Cryogenic Applications, said that in order to test the efficiency of each milling system, they chose three sample materials – a tough tyre rubber with a low glass transition temperature, a polyethylene polypropylene co-polymer, and plasticised PVC which is relatively easy to grind.

‘For each of those materials, the efficiency of each of the cryogenic particle size reduction systems was determined by analyzing a combination of three main attributes of the systems – the production rate (a measure of the weight of product output per hour as a function of energy input), the amount of liquid nitrogen consumed during the grinding process, and the grinding efficiency, or the output yield of particles below a given size,’ said Trembley.

Since the manufacturer’s power descriptions of mills used in the tests were given in different units and the grinding temperatures in the various mills were different, the mill production rate was normalized by the total power used during grinding.

‘For the turbo and pin-mills, the ratio of the amount of nitrogen used to the amount of product produced (the so-called LIN ratio) was calculated assuming that a percentage of the installed power dissipated by the mill was converted to heat. An additional five per cent consumption was added to the calculated value due to pressure and heat transfer losses. Using this formula, the liquid nitrogen consumption was based on the cooling required to cool the material. The grinding efficiency was measured as the single pass yield of material smaller than 180 microns,’ said Trembley.

The results from a number of different cryogenic grinding trials with the various milling systems showed that while the high-speed turbo mill and the pin mill were easily able to achieve between 80-100 per cent yields of 180 micron particle sizes for some of the materials, the hammer mill and the attrition mill struggled to achieve 30 per cent, even though they used less liquid nitrogen to produce a specific production rate of material compared with the turbo and pin mills.

In terms of the specific production rate, which is measured in kilograms of material per kilowatt hour of installed power, the results showed that both the hammer mill and the attrition mill also required considerably more power to achieve the same throughout as the pin and turbo mills.

The researchers also found that while the yield of sub 180 micron particles were comparable for both the pin mill and the turbo mill, the pin mill used more liquid nitrogen in the milling process due to the heat generated from the two rotating surfaces on the mill.

‘The ratio of how many kilograms of nitrogen are needed to produce one kilogram of product is an important criteria in making a selection of what sort of mill to use for a particular process. Our analysis showed that even though the yield of the pin mill was similar to that of the turbo mill, the nitrogen used to generate the equivalent amount of material was almost twice as much in some cases,’ said Trembley.

The turbo mill, it transpired, was superior to all the other mills that were tested based on the combination of specific energy input, LIN ratio and fine particle yield, being able to produce a higher percentage of finer particles.

Trembley believes that the future widespread use of such mills will provide processors with a more economical solution to produce a wide range of materials than many of the more traditional milling systems in use today.