Title of Invention

CRYOPULVERIZER AND A METHOD OF CRYOGRINDING

Abstract

Cryogrinding is a useful technology for recycling of waste plastics for industrial application. Cryogrinding of waste plastics has lot of advantages over conventional grinding because of the brittle fracture of these materials at low temperatures. A fully indigenous cryopulverizer has been developed for cryogrinding of waste plastics and other difficult to grind materials. The cryopulverizer mainly consists of a filling hopper, an agitator drum provided with the low level and high level sensors, screw feeder operated by a variable frequency drive for controlled feeding of the raw material, screw cooler with controlled speed for cooling the raw material to the desired low temperature, impact mill with integral blower and classifier, cyclone separator for efficient separation of the ground products and the cold nitrogen vapour, liquid nitrogen supply system with safety arrangements and a drive system for the mill. The bearings and shaft assembly of this cryopulverizer has been specially designed for easy and reliable operation. Nitrogen gas at ambient temperature gets injected around the gland seals of the mill shaft so that no moisture from the ambient is sucked into the grinding zone of the cryopulverizer. The sealing of the shaft during low temperature grinding is quite important as the hammers get clogged if moisture enters the grinding zone. Control supply of LN2 for optimal cooling of the raw materials is done as set by a PID controller by switching the solenoid valves in the LN2 supply line. LN2 consumption per unit quantity of raw material is minimized by pre-cooling the raw material in the bucket elevator with waste cold nitrogen vapour from the cryopulverizer flowing in counter flow direction. Apart from waste plastics, the above cryopulverizer will be quite useful for size reduction of many materials that are difficult to grind by conventional techniques.

Full Text

FIELD OF INVENTION: This invention relates to a cryopulverizer and a method of cryogrinding of waste plastics and other difficult to grind materials. More particularly this invention relates to cryogrinding of waste plastics in a fully indigenously developed cryopulverizer. Cryogrinding is a process in which the material to be ground (example: waste plastics/PVC scraps) is cooled to the desired low temperature (below the glass transition temperature) by means of a cryogenic fluid such as liquid nitrogen (LN2) and ground in a low temperature compatible mill. Cryogrinding is a useful technology for recycling of waste plastics for industrial application. Cryogrinding of waste plastics has lot of advantages over conventional grinding because of brittle fracture of these materials at low temperatures. Some of the significant advantages of cryogrinding are no chemical degradation of the product, finer particle size, higher throughput, lower energy consumption, no clogging and gumming of the mill, inert atmosphere to prevent explosion in the grinding chamber, etc. This technology can also be used to grind other materials that are difficult to process by conventional grinding. DESCRIPTION OF PRIOR ART: The basic principle of embrittlement of materials such as waste plastics for cryogrinding is cooling the material below its glass transition temperature. Two U.S. patents, Patent no. 4102503 by Meinass m July, 1978 and Patent no. 4116017 by Oberpriller in September, 1978 has been obtained for methods and apparatus for low temperature milling of materials and low temperature cooling of materials respectively. Meinass has discussed a process for cold embrittlement of materials by chilling the materials to a temperature sufficiently low to embrittle it by passing it in direct heat exchange with a cooling gas and entrain the chilled material through a mill in a carrier gas which can be of same composition as the cooling gas. Oberpriller has discussed the embrittlement of the material to low temperature with a circulated cooling gas which is itself cooled by injection of an expandable coolant in liquid form therein. M/s Hosokawa Alpine, Germany has developed cryogenic pulverizers, which can be operated down to -40°C. LIMITATIONS: The patents available for scrutiny and the technical information from M/s Hosokawa Alpine, Germany clearly show that the type of pulverizing systems reported therein can not be operated down to liquid nitrogen temperature (-196°C). Further they have different type of mill temperature control system, which is not suitable for dynamic control of the grinding zone temperature. Further, they are prone for suction of moist air into the low temperature region of the mill causing clogging of the mill and hammers and thus improper functioning of the cryopulverizer. The patented cryopulverizers have no provision to utilize the waste cold nitrogen vapour exiting from the mill. OBJECTS OF THE PRESENT INVENTION: The present invention relates to cryopulverizer for cryogrinding of waste plastics by embrittling the material below its glass transition temperature by liquid nitrogen spray and also a method of cryogrinding using the said cryopulverizer. It is another object of the invention to invent a cryopulverizer whose bearings and shaft assembly are specially designed and operate at almost ambient temperature at the bearing area where as the mill is operated at very low temperature down to -196°C. The bearings are self aligned in nature and are of heavy-duty type for continuous operation. It is another object of the invention means are provided to inject nitrogen gas at ambient temperature aroimd the gland seals of the mill shaft so that no moisture from the ambient is sucked into the grinding zone of the cryopulverizer during low temperature grinding as well as during warming up period of the mill after low temperature grinding. The sealing of the shaft during low temperature grinding is quite important, as the hammers get clogged if moisture enters the grinding zone. It is another object of the invention to use a PID (Proportional Integral Differential) controller for controlling LN2 supply for optimal cooling of the raw material before entering into the grinding zone and maintaining the grinding zone temperature. A sensor mounted at the exhaust of the built in blower in the mill dynamically measures the temperature of the ground product and the cold vapour, gives feed back to the PID controller for efficient control over the supply of LN2. It is another object of the invention to use the cold nitrogen vapour from the exit of the cyclone separator for pre-cooling the raw materials by utilizing the enthalpy available in the cold nitrogen vapour and recirculating part of this cold nitrogen vapour to the grinding zone of the mill to minimize liquid nitrogen consumption for the process. BRIEF DESCRIPTION OF THE DRAWINGS: The above and other objects, feature and advantages of the present invention will now be described with reference to the accompanying drawings in which: Fig. 1 is the schematic diagram of the cryopulverizer; and Fig. 2 is the schematic diagram of the shaft seal assembly of the cryopulverizer. SPECIFIC DESCRIPTION: The cryopulverizer mainly consists of a filling hopper, an agitator drum provided with the low level and high level sensors, screw feeder operated by a variable frequency drive for controlled feeding of the raw material, screw cooler with controlled speed for cooling the raw material to the desired low temperature, impact mill with integral blower and classifier, cyclone separator for efficient separation of the ground products and the cold nitrogen vapour, liquid nitrogen supply system with safety arrangements and a drive system for the mill. As shown in Fig. 1, the material to be ground is transported from the filling hopper (1) to the screw cooler (4) via the agitator drum (2) and the screw feeder (3). The material is cooled to the desired low temperature as set by the PID controller in the screw cooler (4) by liquid nitrogen spray (5). Liquid nitrogen from the storage container is supplied through insulated transfer line. Embrittle material from the screw cooler (4) enters to the cryopulverizer (6) where it is ground to fine particles as per the classifier (7) adjustment in the mill. The ground product along with cold nitrogen vapour from the mill is fed to the cyclone separator (9) where the ground product (10) is collected at the bottom of the cyclone separator and the cold nitrogen vapour (11) at the top of the cyclone separator. Part of the cold nitrogen vapour from the cyclone separator is fed back to the grinding zone and the rest is used to pre-cool the raw material. A temperature sensor (12) mounted at the exhaust of the built in blower (8) in the mill is connected to the PID controller for regulating the liquid nitrogen supply by controlling the solenoid valves in the liquid nitrogen supply line. The raw material is pre-cooled in the bucket elevator type conveyor (13) wherein cold nitrogen vapour from the exit of the cyclone separator flows in counter flow direction of the material travel through buckets (14). As shown in Fig. 2, ambient temperature nitrogen gas is injected through six numbers of copper capillary tubes (2) around the gland seal (1) while the cryopulverizer is in operation. I CLAM: 1. A cryopulverizer for cryogrinding of waste plastics which comprises of: a) Means to supply liquid nitrogen for cooling the raw material down to -196°C or to a temperature within this limit as required; b) Means for cooling the raw material below its glass transition temperature by liquid nitrogen spray, c) Means for grinding these brittle materials at the desired temperature down to-196°C in the low temperature; d) Cyclone separator for separating the ground products from cold nitrogen vapour e) Means to utilize the cold nitrogen vapour from the cyclone separator for cooling the grinding zone as well as pre-cooling the raw material; and f) Ventilating means for venting the warm nitrogen vapour to atmosphere. 2, The cryopulverizer as claimed in claim 1, wherein the means to supply The nitrogen gas injected at a pressure of about 1.5 bar through is a set of six numbers of copper capillary tubes symmetrically located. 3. The cryopulverizer as claimed in claim 1 or 2, wherein the supply of the liquid nitrogen spray at a pressure of about 1.5 bar to the raw material is controlled by the PID controller, which switches the solenoid valves. 4, The cryopulverizer as claimed in claim 1 or 2, wherein the means for cooling is a temperature sensor mounted in the exhaust line of the blower where the ground product along with cold nitrogen vapour is carried over to the cyclone separator, which dynamically controls the desired low temperature of the system by giving input to the PID controller, the optimum PID values being stored for standard default operation of the mill during subsequent operations. 5. The cryopulverizer as claimed in any one of the preceeding claims, wherein the means for grinding is a suitable mill such as an impact mill. 6. The cryopulverizer as claimed in any one of the preceeding claims, wherein means for injecting ambient temperature nitrogen gas is provided around the gland seals of the mill shaft for preventing suction of the moisture into the mill thereby clogging of the mill hammers 7. The cryopulverizer as claimed in any one of the preceeding claims, wherein the bearings and shaft assembly being specially designed and operate almost at ambient temperature at the bearing area whereas the mill operates at very low temperature, the said bearings are self aligned in nature and are of heavy-duty type for continuous operation and are positioned outside of the low temperature zone for easy and reliable operation. 8. The cryopulverizer as claimed in any one of the preceeding claims, including a raw material pre-cooling bucket elevator system which enables the raw material to be pre-cooled using the waste cold nitrogen vapour from the cryopulverizer flowing in the counter flow direction to enable pre-cooling the raw material and reduction of the amount of LN2 consumption per unit quantity of raw material. 9. A method for cryogrinding of waste plastics which comprises the steps of: a) supplying liquid nitrogen for cooling the raw material down to -196'C or to a temperature within this limit as reqmred; b) cooling the raw material below its glass transition temperature by liquid nitrogen spray, c) grinding these brittle materials at the desired temperature down to -196'C in the low temperature compatible mill; d) separating the ground products from cold nitrogen vapour in the cyclone separator; e) utilizing the cold nitrogen vapour from the cyclone separator for cooling the grinding zone as well as pre-cooling the raw material; f) venting the warm nitrogen vapour to atmosphere. 10. The method as claimed in claim 9, wherein the liquid nitrogen spray at a pressure of about 1.5 bar to the raw material is controlled by the PID controller, which switches the solenoid valves and hence the liquid nitrogen supply as per the system requirement. 11. The method as claimed in claims 9 or 10, wherein ambient temperature nitrogen gas being injected around the gland seals of the mill shaft for preventing suction of the moisture into the mill thereby clogging of the mill hammers. 12. The method as claimed in any one of claims 9 to 11, including the step of pre- cooling the raw material using the waste cold nitrogen vapour from the cryopulverizer flowing in the counter flow direction.

Documents:

064-che-2004-abstract.pdf

064-che-2004-claims filed.pdf

064-che-2004-claims granted.pdf

064-che-2004-correspondnece-others.pdf

064-che-2004-correspondnece-po.pdf

064-che-2004-description(complete) filed.pdf

064-che-2004-description(complete) granted.pdf

064-che-2004-drawings.pdf

064-che-2004-form 1.pdf

064-che-2004-form 13.pdf

064-che-2004-form 26.pdf

064-che-2004-form 3.pdf