EP1449590A1

EP1449590A1 - Improved method and device for treating waste

Info

Publication number: EP1449590A1
Application number: EP03076962A
Authority: EP
Inventors: designation of the inventor has not yet been filed The
Original assignee: SITA Ecoservice Nederland BV
Current assignee: Prezero Special Waste BV
Priority date: 2003-06-23
Filing date: 2003-06-23
Publication date: 2004-08-25

Abstract

The present invention relates to a method and device for treating a waste stream comprising a liquid fase which is at least partially associated to solid materials. The method according to the invention comprises the steps of: (i) subjecting said waste to a size reducing treatment, (ii) separating the size reduced waste in a first fraction with an elevated metal content and a second fraction with a reduced metal content, (iii) subjecting the first fraction to a cold treatment thus forming a cold waste fraction, (iv) separating the cold waste fraction in a third fraction with an elevated metal content and a fourth fraction with a reduced metal content. The device according to the invention is useful for carrying out such method.

Description

The present invention relates to a method and device for treating waste. More particular the method and device can be used for treating a waste stream comprising a liquid phase which is at least partially associated to solid materials. EP 0 480 508 discloses a method and device for treating a waste comprising a solid phase and a liquid phase of product remnants, which partially adhere to the solid phase. The method disclosed comprises the steps of shredding the waste, separating liquid and subjecting the separated solid constituents to a cold treatment. By cooling the shredded waste to below the glass temperature (Tg) the product remnants become solid and brittle and thus can easily be removed from metal surfaces. Therefore, after the cold treatment the material is vibrated with an impactor and ferromagnetic metals are separated by means of a magnetic separation technique.
Dutch patent application 9 001 596 discloses a method for separating paint material associated to tin originating from the packaging material. According to the method the material is cooled in a liquid nitrogen bath, where after the cooled material is transported to a sledge hammer and subsequently materials are separated by means of screening and with the use of a magnetic separation technique.
Thus, in the methods according to the art the entire solid part of the waste stream with product remnants associated thereto is subjected to a cold treatment. Since the cold treatment is mainly necessary for separating metal constituents, constituenst with liquid, including viscous, consistency associated therewith, there is an unnecessary consumption of coolant for cooling nonmetal solid constituents and especially liquids associated to these non-metal solids. As consumption of coolant during the cold treatment substantially contributes to the overall energy consumption of the waste treatment process it is an object of the present invention to provide a method and a device for treating waste, in which the consumption of coolant during the cold treatment is reduced. This can be achieved by separating the waste before the cold treatment in a fraction with an elevated metal content and a fraction with a reduced metal content and subsequently subjecting the fraction with an elevated metal content to the cold treatment.
Therefore the method according to the invention for treating waste is characterized in that it comprises the steps of:
i) subjecting a waste comprising solid metal constituents to a size reducing treatment,
ii) separating the size reduced waste in a first fraction with an elevated metal content and a second fraction with a reduced metal content,
iii) subjecting the first fraction to a cold treatment thus forming a cold waste fraction,
iv) separating the cold waste fraction in a third fraction with an elevated metal content and a fourth fraction with a reduced metal content.
The device according to the invention for treating waste is characterized in that it comprises:
i) a number of units for reducing the size of solid waste particles,
ii) a first unit for separating size reduced waste in a first fraction with an elevated metal content and a second fraction with a reduced metal content,
iii) a cold treatment unit for cold treatment of the first waste fraction,
iv) a second unit for separating size reduced cold waste fraction in a third fraction with an elevated metal content and a fourth fraction with a reduced metal content.
In the method according to the invention the volume and/or weight of the solid part of the waste subjected to the cold treatment is reduced. It has been found that associated with this reduction in volume and/or weight of the solid part for cold treatment there is a reduction in the volume and/or weight of material of liquid to highly viscous consistency at ambient temperature associated to solid particles for cold treatment. Thus the method and device according to the present invention have a reduced consumption of coolant per unit treated waste. The material of liquid to highly viscous consistency at ambient temperature may be associated to the solid constituents of the waste stream simply by the fact that it is present in the same fraction. In addition to that it may be at least partially adhered to the solid constituents.
It has been found that in the cold treatment most energy is exchanged from materials that undergo a phase transition e.g. materials with a liquid to (highly) viscous consistency at ambient temperature. Thus reducing the amount of such materials in the waste fraction subjected to cold treatment has a marked effect on the consumption of coolant.
The waste for treatment with the method and device according to the invention comprises packaging materials, such as metals, including ferromagnetic and non-ferromagnetic metals, plastics, glass and ceramic materials, product remnants originally packed in the above mentioned packing materials which can have a solid, liquid or viscous to highly viscous consistency at ambient temperature, such as paint, ink, glue, mastic, oil, highly viscous organic products, in addition organic and watery liquids and finally miscellaneous including materials originating for example from the packaging material of the product remnants including plastics, wood, fibres, textiles, ferrous and non-ferrous metals.
In the method according to the invention the waste is first subjected to a treatment which reduces the size of solid constituents contained in the waste. Such treatment can be performed by shredding the waste in a number, meaning one or more, of shredding devices in order to ensure sufficient size reduction of the waste. Aim of the size reducing treatment is to optimize the separation in the first and second fraction. In general the average particle size needs to be balanced. Too small a particle size requires extensive energy input and causes extensive wear and tear of the equipment. On the other hand the presence of large solid particles in the waste stream might complicate the separation of the first and second fraction. A suitable selection of the average particle size is within reach of the knowledge of the artisan.
Separating the size reduced waste in a first fraction with elevated metal content and a second fraction with reduced metal content can be performed with methods known in the art. The elevated or reduced metal content of the first and second fraction is relative to the metal content of the solid part of the size reduced waste obtained in (i). A preferred aim of the separation in the first and second fraction is to maximize the separation of metal solids comprised in the size reduced waste between the first and second fraction. Suitable separation techniques may include air separation, screening, ballistic methods or may be based on the floatation/deposition of the waste constituents in a liquid medium. Preferably use is made of a magnet to achieve the desired separation. The separation may be based on a single technique or combinations of separation techniques may be used.
The cold treatment is performed by techniques known in the art. Suitable coolants comprise liquid nitrogen, solid CO₂ or a mixture of acetone and solid CO₂. Preferably a liquid coolant is used since this allows an optimum heat transfer from the waste to the liquid coolant. A preferred liquid coolant is liquid nitrogen. If the liquid coolant is a liquid inert gas this can upon evaporation during the cold treatment be used to create an inert atmosphere in the system, preventing explosions and fires. In the cold treatment optionally cool gas produced by the cold treatment of the waste fraction can be used for pre-cooling of the waste fraction, as described in Dutch patent application 9 001 596.
Suitable ways for separating the cold waste fraction in a third fraction with an elevated metal content and a fourth fraction with reduced metal content are known in the art and may be selected as presented for the separation in the first and second fraction. The elevated or reduced metal content of the third and fourth fraction is relative to the metal content of the cold waste fraction obtained in (iii). A preferred aim of the separation in the third and fourth fraction is to maximize the separation of metal solids comprised in the cold waste fraction between the third and fourth fraction.
Preferably the material of the cold waste fraction is impacted before the separation in the third and fourth fraction. By impacting the material the interaction of solidified liquid constituents associated to the solid metal parts is further reduced. The cold waste fraction may be impacted under influence of vibration or by use of a sledge hammer or a hammer mill. Reducing the material in size as described in Dutch patent application NL 9 301 708 is also considered a possible means for impacting the material of the cold waste fraction.
It has been found that in order to reduce the amount of materials with a liquid to highly viscous consistency associated to the metal constituents of the first fraction it can be beneficial to add an absorbing additive to the size reduced waste, before separation in the first and second fraction. Such an absorbing additive presents an additional surface to which materials with a liquid to highly viscous consistency at ambient temperature, such as product remnants, may adhere and thus competes with the metal surface for adsorption of such materials. Thus by adding an absorbing additive adherence of product remnants to metal constituents of the waste is reduced. This facilitates reducing the association of materials with a liquid, including viscous, consitentency at ambient temperature with metal solids e.g. by separating the waste in the first and second fraction. As it has been found that in the cold treatment most energy is exchanged from materials that undergo a phase transition e.g. materials with a liquid to (highly) viscous consistency at ambient temperature this further reduces the consumption of coolant.
Another aspect that is negatively influenced by the presence of materials with a liquid to highly viscous consistency at ambient temperature is down time of the process line. It has been found that such materials contribute to a large extent to fouling of the process equipment. Extensive fouling of the process equipment necessitates maintenance activities, which often result in a down time of the process line. Thus reducing the amount of such materials in at least part of the process line, such as parts especially susceptible to pollution by these materials, among others the cold treatment unit, reduces maintenance requirements and down time of the process line.
In addition to that fluctuating levels of materials with a liquid to highly viscous consistency at ambient temperature make temperature control during cold treatment difficult. Temperature elevations outside the process parameter values may also cause down time of the process line. In addition to that temperature elevations outside the process parameters decrease the quality of cleaning of the metal product. By reducing the amount of materials with liquid to highly viscous consistency at ambient temperature better temperature control of the process is possible.
A further possible benefit provided by addition of an absorbing additive is an increase of the flash point of materials with a low flash point comprised in the waste stream. Increasing the flash point in this way reduces risk of possible explosions and/or fires both in the device and containers used for transport of the processed waste flow.
Another possibility to increase the flash point of the waste stream is by addition of water. The use of water however is limited as it reduces the caloric value of the waste product. In addition to that water is of liquid consistency and thus increases the consumption of coolant if added before the cooling treatment.
A function of the absorbing additive is to provide a surface which competes with the surface of metal material for adherence of materials with a liquid to highly viscous consistency at ambient temperature. In addition to that absorbtion of the liquid, including viscous, material facilitates reducing the association of these materials with metal solids e.g. by separation in the first and second fraction. Suitable absorbing additives may comprise cellulose comprising materials such as paper, cardboard or wood like materials for example waste from sawmills, e.g. sawdust. Alternatively powdered pur foam can be suitable. Preferably the absorbing additive has a caloric value which renders it suitable to be burnt together with non-metal constituents of the waste.
The absorbing additive may be added to the waste before, during of after the size reducing treatment, as long as it is added before separating the waste in the first fraction with elevated metal content and the second fraction with reduced metal content.
The amount of absorbing additive used depends on the properties of the employed additive in relation to the effect that one wishes to obtain. In addition to that it must be considered that at present usually costs for waste burning are calculated on a weight basis. Thus the amount of absorbing additive depends on different deliberations.
In a preferred embodiment of the method according to the invention the fourth fraction and the second fraction are combined. Since the fourth en second fraction both comprise combustible material such a fraction is suitable as a fuel.
In a further preferred embodiment of the invention liquid is separated from the size reduced waste before the cold treatment. The separated liquid is preferably a free flowing liquid. Separation of liquid can reduces the amount of absorbing additive necessary. Thus although it can be used alone in the present invention, it is preferable performed together with the use of an absorbing additive. Also because it has been found that with the separation of liquid alone a relatively large amount of materials with liquid to highly viscous consistency at ambient temperature remains associated with the solid metals, compared to the situation where both methods are used. Reduction of the amount of materials with liquid to highly viscous consistency at ambient temperature has the abovementioned benefits. Liquid is separated while or after reducing the solid waste particles in size, since most material of liquid to highly viscous consistency at ambient temperature is then discharged from its containers. Preferably liquid is separated before addition of an absorbing additive, since then these operations can act complementary in reducing the amount of materials with liquid to highly viscous consistency at ambient temperature.
The method and device according to the invention will be further elucidated by the description of the following nonlimitative embodiment wherein reference is made to the annexed drawing, in which: figure 1 is a schematic overview of a device according to the invention. The full figure is presented by figures 1A and 1B. Interconnecting parts of the device are indicated with identical roman numerals.
Figure 1 shows a device (1) according to the invention for treating waste, which is stored in a bulk bunker (2) or containers (3) of approximately 200 l, from which it is transported by means of transport crane (4) or conveyer belt (5) to inlet lock (6). The waste comprised metal such as paint cans of steel and tin, paint remnants such as paint cakes, pigments, inks and dyes, solvents, such as water and hydrocarbons, plastics, such as plastic paint buckets and compound, highly viscous organic substances, such as oils, glue and mastic, and miscellaneous including paint brushes, paint rollers and fabric. It is the object to separate as much metal from non-metal material both in the overall process and before cold treatment.
From the inlet lock (6) the waste is transported to a closed system, which comprises several nitrogen locks (7) and explosion exhausts (8). The explosion exhausts (8) serve to release pressure in case explosions occur in the system. To further reduce changes of explosions in the system, oxygen tension in the atmosphere is kept low (< 12% O2) among others by constant gas drain by ventilator (9) and introduction of nitrogen gas.
Conveyer belt (10) transports the waste to a first shredder unit (15) for size reduction of the waste. The shredder unit (15) comprises devices (16) for pressing material between the claws located on the rotating units (17) of the shredder unit (15) in case this should be necessary. The shredder unit (15) comprises a hatch operated by device (14), via which any material not capable of entering the shredder unit may be removed.
As absorbing additive sawdust is added to the shredded waste via sawdust inlet (17) which connects to inlet pipe (18) sealed by several nitrogen locks (7). Inlet pipe (18) leads to transport screw (19) which transports the sawdust to the outlet of the first shredder unit (15). Sawdust together with shredded waste is transported by transport screw (20) to the inlet of a second optional shredder unit (21) where the size of particular material comprised in the waste is further reduced. From the outlet of the second shredder unit (21) the size reduced waste is transported by conveyer belt (22) along a magnet (23) which attracts and deposits magnetic material on conveyer belt (24). The material deposited on conveyer belt (24) has an elevated metal content relative to the material transported on conveyer belt (22) and comprises magnetic metal material with non-magnetic material associated thereto.
Non-magnetic material not collected by magnet (23) falls into waste channel (30) from where it is transported via waste screws (31) and (32) to containers (33, 34). The material collected in waste channel (30) comprises a reduced metal content relative to the material transported by conveyer belt (22). By selecting the rotation direction of waste screw (32) the waste can selectively be transported to either container (33) or container (34).
The waste fraction with an elevated metal content transported by conveyer belt (24) is carried to a cold treatment unit (35). The cold treatment unit comprises a rotating drum (36) inclining downward in down stream direction and provided with mixing members (not shown).
Liquid nitrogen (the liquid coolant) is introduced via conduits connected to piping (37) transporting the liquid nitrogen from nitrogen storage (38). Nitrogen gas emitting from the cold treatment unit (35) is removed via nitrogen exhaust (39) connected to nitrogen ventilator (40).
The liquid nitrogen is guided in a flow counter to the shredded material for cooling and is discharged via nitrogen exhaust (39) provided in the inlet (41) of the cold unit (35) and connected with ventilator (40). The inlet (41) of the cold unit (35) comprises a valve (42) for the purpose of enabling discharge of shredded material to a container (43) in case of emergency situations.
In the rotating drum the size reduced shredded waste is cooled to below the glass temperature of paint and/or glue and/or ink and/or mastic remnants present in the waste.
The still cold shredded material passes a hammer mill (44) which separates metal and adhering material in frozen or glass state by impacting on it. The material is subsequently transported by conveyer belt (45) which carries the still cold shredded material past a second magnet (46) which attracts and then deposit the magnetic material on a conveyer belt (47) for separated magnetic material. Because now the waste stream is reduced in size magnet (46) has a smaller capacity then the first magnet (23). From conveyer belt (47). The material is subsequently transported to conveyer belt (48) and (49) and to container (50) or (51) depending on the direction of conveyer belt (49).
The material transported by conveyer belt (47) is elevated in metal content relative to the material transported by conveyer belt (45). It comprises mostly of clean magnetic metal material.
The material with a reduced metal content is transported by conveyer belt (52) and is transported from thereon to conveyer belt (10). The purpose of this is redirection of this material to either container (33) or (34), thus mixing the two fractions not collected by the magnets. In the given situation of the plant plot redirection of the material via conveyer (10) is more convenient than direct redirection to either container (33 or 34). In a differing situation direct transport to container (33) or (34) might be more convenient.

Claims

Method for treating a waste stream comprising a solid metal constituents with product remnants of liquid to highly viscous consistency at ambient temperature associated thereto which method comprises the steps of:

i) subjecting a waste comprising solids to a size reducing treatment,

ii) separating the size reduced waste in a first fraction with an elevated metal content and a second fraction with a reduced metal content,

iii) subjecting the first fraction to a cold treatment thus forming a cold waste fraction,

iv) separating the cold waste fraction in a third fraction with an elevated metal content and a fourth fraction with a reduced metal content.
Method as claimed in claim 1, wherein an absorbing additive, such as a cellulose comprising material for example a material selected from the group comprising paper, cardboard and sawdust, or powdered pur foam is added before step (ii).
Method as claimed in any of the claims 1-2, wherein the solid metal fraction comprises ferromagnetic metals and the separation in the first and second fraction is carried out by use of a magnet.
Method as claimed in any of the claims 1-3, wherein the fourth fraction and the second fraction are combined.
Method as claimed in any of the claims 1-4, wherein liquid is separated prior to step (ii).
Device for treating a waste stream comprising a solid metal fraction with product remnants of liquid to highly viscous consistency at ambient temperature associated thereto, which device comprises:

i) a unit for reducing the size of solid waste particles,

ii) a first unit for separating size reduced waste in a first fraction with an elevated metal content and a second fraction with a reduced metal content,

iii) a cold treatment unit for cold treatment of the first waste fraction,

iv) a second unit for separating size reduced cold waste fraction in a third fraction with an elevated metal content and a fourth fraction with a reduced metal content.
Device as claimed in claim 6, further comprising a unit for adding an absorbing additive to the waste.
Device as claimed in any of the claims 6-7, wherein the means for separating the size reducing waste in a fraction with an elevated metal content and a fraction with a reduced metal content comprises a magnet.
Device as claimed in any of the claims 6-8, further comprising a unit for separating liquid out of the size reduced waste.