In some size reducing equipments including high-speed hammer mills, air classifier mills, jet mills etc, a steady flow of ambient air is enough to dissipate the heat generated during grinding. The air flow is typically provided by the mill’s pumping action or supplemented by an auxiliary blower. The air volume required to dissipate the heat is based on the mill’s size, how much heat it generates, and the mill’s capacity for pumping the air. But for some applications, additional cooling is required. Providing the additional cooling whether to the material or the mill can be an expensive and complicated task.
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Using cryogenic fluids to cool the grinding process can provide many benefits, including
1 Protective material quality,
2 Improving material characteristics,
3 Providing controllable cooling,
4 Enhancing processing,
5 Eliminating safety and environmental hazards, and
6 Minimizing equipment maintenance.
Cryogenics is the study of very low temperatures or the production of the same.
The word "cryogenics" comes from two Greek words; "kryos", which means cold
or freezing, and "genes" meaning born or produced.
Liquid nitrogen is the most widely used cryogenic grinding fluid.
Properties of Liquid Nitrogen
Liquid nitrogen is inert, colourless, odorless, non-corrosive, nonflammable, and extremely cold. Nitrogen makes up the major portion of the atmosphere (78.03% by volume, 75.5% by weight).
B. Physical Properties
1 Molecular Weight : 28.01
2 Boiling Point @ 1 atm : -195.8°C, 77oK
3 Freezing Point @ 1 atm : -210.0°C, 63oK
4 Critical Temperature : -146.9°C
5 Critical Pressure : 33.5 atm
6 Specific Gravity, Gas (air=1) @ (20°C), 1 atm : 0.967
7 Specific Gravity, Liquid (water=1) @ (20°C), 1 atm : 0.808
8 Expansion Ratio, Liquid to Gas, BP to 68°F (20°C) : 1 to 694
C. Health Effects
Although nitrogen is nontoxic and inert, it can act as a simple asphyxiant by displacing the oxygen in air to levels below that required to support life. Inhalation of nitrogen in excessive amounts can cause dizziness, nausea, vomiting, loss of consciousness, and death. Death may result from errors in judgment, confusion, or loss of consciousness that prevents self-rescue. At low oxygen concentrations, unconsciousness and death may occur in seconds and without warning. Personnel, including rescue workers, should not enter areas where the oxygen concentration is below 19.5%, unless provided with a self-contained breathing apparatus or air-line respirator.
Minimum effective temperature for some common refrigerants.
Grinding quickly, grinding cool
GRINDING is a pretty inefficient process: up to 99% of the mechanical energy entering the mill ends up as heat. With industrial mills typically in the size range 20-100 kW, cooling is a significant issue - and for materials that deform or melt when they are warmed, temperature rise can also be a problem. Water or other liquids provide effective heat transfer, but not all materials are suitable for wet grinding. Indirect water cooling
of the mill is of limited effectiveness because of the lack of heat transfer area. Most dry mills therefore rely on a large flow of air or nitrogen to both cool and transport the product. But even with a large airflow, some particles can reach temperatures of up to
300°C, which is often high enough to cause a significant loss of quality.A clean and effective way to boost cooling is to inject liquid nitrogen at a temperature of -196°C into the product upstream of the grinding process. An example is the Cryo-grind system
offered by industrial gas supplier Air Products.
Grinding processes using liquid nitrogen
Grinding processes using liquid nitrogen fall into two types.
i. Temperature-controlled grinding
ii. True cryogenic grinding
In Temperature-controlled grinding, the liquid nitrogen is injected directly into the mill, where it acts as a heat transfer medium rather than a refrigerant. Liquid nitrogen injection rate is controlled by a sensor that measures the temperature of the air and nitrogen leaving the mill. The ground product leaves the mill at typically 10-30°C, and at no stage does its temperature fall low enough to cause embrittlement.
True cryogenic grinding is a different process designed to exploit the tendency of many materials to become brittle at low temperatures. In grinding plastics and rubbers, for example, the stresses needed to break up the material are dissipated by relaxation. As a result, plastics need 10 - 100 times as much energy to grind as inorganic materials, or typically 100 - 1000 kWh/t at room temperature for particles in the size range 100-1000 µm. Rapid impacts and low temperatures hinder the ability of polymers to relax. The effect is pronounced: for some grades of polypropylene, cooling from 20°C to -20°C increases stresses by an amount corresponding to a million-fold decrease in contact time during the impact. This is where cryogenic grinding comes in. To allow time for the material’s temperature to fall sufficiently, the liquid nitrogen is sprayed onto the feed in a
special contactor, such as a screw conveyor, well upstream of the mill. Large particles are harder to chill, so the feed is typically a granulate of around 3-6 mm size. Cooling shrinks the crystal lattice of the substance to be ground, and introduces microscopic cracks that greatly reduce the amount of energy needed to cause fracture. Usefully, the
heat capacity of the material decreases as the temperature falls, thus reducing the amount of liquid nitrogen needed to reduce the temperature further. In its glassy state the material is easier to grind, so throughput can be increased, particle size can be reduced, or both.
Wear on the mill is less, and the presence of inert nitrogen usually keeps the oxygen level below 6%, so providing useful protection against dust explosions. As well as being free from damage caused by temperature excursions, the cryogenically-ground product typically has a narrower particle size distribution than can be obtained from grinding at ambient temperature. The product generally flows more freely, and the mill is easier to clean.
Raw material passing along a conveyor is cooled using controlled amounts of liquid nitrogen which allows for finer grinding and increased throughputs.
Cryo-Grind® Size Reduction Systems
For fine grinding plastics and pigments
Air Products Cryo-Grind® systems perform three essential tasks in the efficient particle size reduction of all types of plastics and pigments:
Grinding tough, thermoplastic materials- polypropylene, nylon etc. - at cryogenic temperatures ensures that plastics are brittle enough to be ground below 300 micron particle size: and even down to 50 micron and below with the improved new Cryo-Grind® systems.
Materials normally brittle enough to grind at ambient temperatures can start to degrade when the ambient temperature rises i.e. in summer, or if throughput increases are required .Cryo-Grind® systems inject controlled amounts of liquid nitrogen to regulate the heat of the grinding mill and allow higher material throughputs at any time
of the year.
Organic materials (especially pigments) have an increased risk of explosion when very fine particles combine with air and the heat of the grinding mill. The use of nitrogen to make the atmosphere inert reduces the possibility of this occurrence.
Factors affect liquid nitrogen consumption in cryogenic grinding
• Smaller particles
Plastics and pigments can be introduced into high performance materials with improved mixing and better final material quality.
• Regular particle size
Cryogenically ground materials have an arrower size differential and smoother surface area
• Efficient process
Nitrogen usage is minimized and throughput maximized by using Air Products Cryo-Grind® systems
Fig. 2 shows a typical cryogenic liquid cylinder. Cryogenic liquid cylinders are insulated, vacuum-jacketed pressure vessels. They come equipped with safety relief valves and rupture discs to protect the cylinders from pressure build-up. These containers operate at pressures up to 350 psig and have capacities between 80 and 450 liters of liquid. Product may be withdrawn as a gas by passing liquid through an internal vaporizer or as a liquid under its own vapor pressure.
So what’s best?
Not surprisingly, the best grinding technique depends on the product and the process. For easy-to-grind products such as many inorganic substances, especially with large particle sizes, cryogenic grinding may not be economic
Air Products Cryo-Grind™ systems enable rubber scrap materials to be ground into fine particles for re-use in compounding for new rubber and plastic products.
1 Can grind smaller rubber particles down to below 200 micron
2 Regular particle size
3 Efficient process by introducing minimum nitrogen and maximum throughput
4 Improved surface morphology
Cryogenic grinding of Herbs
A scientifically controlled study using four herbs was conducted at Frontier Herbs in the Fall of 1996, comparing cryogenic grinding methods with normal grinding methods. The herbs tested included Feverfew, Goldenseal, Valerian and Echinacea. In all cases the cryogenically ground herb contained greater amounts of the constituents tested. Feverfew herb showed the greatest difference, with the cryogenically ground herb containing 21.8% higher levels of parthenolide, the primary active constituent. Valerian root showed an 18.7% increase in valerenic acid when cryogenically ground. Goldenseal root showed a 16.4% increase in berberine and 10.7% increase in hydrastine. Lastly, Echinacea purpurea root showed a 12.1% increase in total phenolic content in the cryogenically ground root. Test results were obtained by HPLC (high performance liquid chromatography) methods.
Cryogenic grinding was shown to significantly affect active constituent levels in herbs. Test results showed an average increase of 15.6% in constituents tested in four medicinal herbs when they were ground cryogenically. The range was 10.7% to 21.8%, indicating that some herbs are affected more than others by the temperatures at which they're ground.
Cryogenic grinding plant for Spices
A dosing wheel delivers the required quantity of spice, e.g. pepper, to the mill. The temperature-controlled supply of liquid nitrogen or carbondioxide ensures the material does not heat up from the heat arising during grinding. Finely pulverized, the material falls through the mill’s sieve and leaves the mill through a cellular wheel sluice .The mill gas is purified in a filter and the excess gas released by a butterfly valve. The remaining gas is returned to the mill. In this way gentle, dry and continuous grinding is possible .The controlled low temperature prevents the product from caking in the mill. The grinding process works continuously and the product retains a maximum of aroma.
Cryogenic grinding data,
e.g. for pepper
Particle size: 700 µm
Production rate: 750 kg/h
consumption: 0.25 kg/kg pepper
Driving power: 35 kW
Cryogenic grinding plant for thermoplastics
A dosing wheel meters the plastic pellets , e.g. PE or PA, into the mill .The grinding heat would normally cause these thermoplastics to melt, rendering them ungrindable. The cold of the liquid nitrogen prevents this by embrittling the material in the cooling conveying screw .The cryogenic ground plastic and the gas drop into a collecting bin. The pulverized product leaves the mill for further processing through a cellular wheel sluice. The mill gas is purified in a filter and a quantity corresponding to the amount of nitrogen fed into the system released. The remaining gas is passed back to the mill for utilization of the residual cold.
Cryogenic grinding data,
e.g. for polyamide
Particle size: 80 µm (d50)
Production rate: 350 kg/h
Consumption: 1.25 kg/kg polyamide
Driving power: 21 kW
Polyamide pellets Cryogenic ground polyamide
Schematic of a cryogenic grinding plant for thermoplastics
Pin Mill for Cryogenic Ultrafine Grinding
Mill operation with liquid nitrogen causes the feed material to become brittle. The specific precrushing energy is reduced meaning that the mill achieves high levels of grinding performance. Additionally the cryogenic mode prevents grinding losses and thermal damage to the feed material that would otherwise be caused by the volatisation or overheating of constituent ingredients. The inert gas atmosphere provides a high degree of safety when grinding flammable and potentially explosive products.