Particle size reduction using supercritical materials

Details Particle size reduction using supercritical materials Particle size reduction using supercritical materials

2000-09-08

2004-01-20

Le, H. Thi (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Including a second component containing structurally defined...

C241S001000, C241S002000, C241S015000, C241S021000, C241S024160, C241S027000, C428S402000, C428S489000, C428S492000

Reexamination Certificate

active

06680110

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the use of supercritical gases for material size reduction and separation. More specifically, this invention relates to swelling a material particle with a supercritical gas and then rapidly dropping the pressure so that rapid internal expansion of the supercritical gas inside the material particle cause the material to be torn apart and further reduced in particle size.
2. Description of the Related Art
In the area of reducing the size of materials from coarse pieces to fine powder, it is generally known to decrease the size of such materials to small particles. For example in the rubber recycling and reusing industry, it is generally known to reduce rubber chips, such as those obtained from the shredding of tires, to particles of irregular outline that pass through a minus 80-mesh or a minus 50-mesh or finer. Wood chips are reduced to a smaller particle size before ligand is extracted. By increasing the surface area of any given material, smaller particle sizes facilitate chemical reactivity and the ease at which a material will dissolve into various mixes.
A variety of techniques for reducing the particle size of materials from coarse materials to fine materials have been developed in the flour, paper pulp industry, paint pigment compounding industries and rubber recycling industries. For example, a variety of rubber products (e.g., natural rubber, synthetic rubber, vulcanized rubber, automotive tire scrap, etc.) may be reduced to coarse materials. Coarse materials are produced from a variety of methods including cutting, shredding, chopping, chipping, milling and grinding. Other known methods include the milling of a material between horizontal grinding stones in a horizontal grinding mill. Such milling techniques include grinding the material between opposed milling wheels, such that one wheel is fixed and the other wheel rotates relative to the fixed wheel. Such known milling techniques include pressing the two wheels against a material slurry, such that the rubber is ground to a fine state (i.e., powder) in a single pass. However, such known milling methods have the disadvantage of creating friction and introducing energy to the slurry, which may increase the temperature of the slurry. Increased slurry temperatures may cause “Slash over” in which the slurry becomes a largely dry material mass that inhibits grinding. Such known milling methods further have the disadvantage of not producing a uniformly fine material powder that passes through a minus 50-mesh. Such techniques also require large amounts of water, which requires large amounts of energy to evaporate or centrifuging. Addition methods for producing particles of a decreased size include cryogenic cracking of the material and the use of bead or mead mills. Moreover, smaller particle sizes are desirable, such as particles in the micron or submicron sizes.
Accordingly, it would be advantageous to have a method for converting a coarse material to a finely ground particle. It would also be advantageous to have a method for converting a coarse material to a finely ground particle without centrifuging or using large quantities or any quantity of water. It would also be advantageous to have a method to material to a decreased size that decreases soak time of the material. It would also be advantageous to have a method to convert a material to a decreased size that decreases grinding time of the material. It would also be advantageous to have a method that converts material to a decreased size carried out at a relatively low temperature of material slurry. It would also be advantageous to have a material that chemically combines with an additive that modifies the physical properties of the material.
SUMMARY OF THE INVENTION
In accordance with the present invention is provided a method for converting a material to a decreased size using supercritical gases. The method includes optionally preparing said material to make said material capable of being dispersed with supercritical gases. Such optional preparation involves reducing the material to a coarse particle size by shredding, chopping, chipping, milling or grinding.
Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The size of a material may be using a gas that is in a supercritical liquid state. According to one embodiment, the gas is supercritical carbon dioxide. The course material for size reduction may be any material that has been previously chopped, shredded, chipped, cut or pulverized milled ground etc. In on embodiment the material is between 2 and 200 mesh. In another embodiment the material is 60 to 150 mesh. In another embodiment the material is −40 to 2 mesh.
In a exemplary embodiment, the material may contain rubber. Examples of rubber include natural rubber, synthetic rubber, recycled rubbers containing polyethylene and/or polypropylene, vulcanized rubber, carbon black, waste from tire production, various polymers, various plastics, thermoplastic elastomers, thermoplastic vulcanates, polyethylene plastics, etc.
Gases with critical pressures at or below 1,100 psi are desirable because less pressure is needed to change the gas into a supercritical state. Higher pressures are disadvantageous because more energy is required, safety considerations and the equipment for generating such high pressures is more expensive. Any refrigerants with critical pressure of less than 1,100 psi are suitable for the present invention. Such refrigerants include freon and the Suva® gases listed in Table 2. Carbon dioxide is also suitable, but has a critical pressure that is higher than most refrigerants. An advantage of carbon dioxide is that the critical temperature is 31° C.
Other materials may include wood chips, wood pulp or sawdust. It is desirable to extract lignin from such coarse wood materials to facilitate a paper making process that is better for the environment. Additional materials suitable for processing include elastomers, plastomers, agricultural materials, biological materials and forest materials, etc.
Coarse materials are introduced into a pressurizing vessel capable of pressurizing a gas at a temperature above freezing into a supercritical state. A gas or liquid is introduced into the vessel and the pressure and/or temperature of the vessel is increased so that the gas or liquid reaches supercritical conditions. In some uses it may be preferable to use a gas or liquid that has an ambient critical temperature. In other uses, the supercritical solvent of interest may have higher critical temperatures. The critical temperature preferably does not exceed 120° C. Many refrigerants, such as most of the Suva® refrigerants, offered by DuPont, have critical temperatures of less than 120° C. and thus can be used for particle size reduction. A listing of suitable Suva® refrigerants is found in Table 2. Examples of such gases include carbon dioxide and refrigerants such as Suva® 95. Gases that are relatively unreactive, non-toxic, and have a critical temperature of less than 120° C. and a critical pressure of less than 1,100 psi are preferred.
The vessel into which the material for size reduction and the gas is introduced is made to operate at pressures sufficiently high to bring about a supercritical state in a gas that has a supercritical temperature of generally less than 120° C. The pressure of the vessel must be sufficiently high so that when the pressure is rapidly released, the particles for size reduction are ripped apart by the rapidly outward expanding pressure. When gases are in a supercritical state, they will have a solvating effect on the coarse material. For example, tire ch

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