METHOD FOR SEPARATING LEAD FROM COATED GLASS

Abstract: The invention relates to a method for separating lead from coated glass, especially glass from discarded picture tubes or similar lead-containing glass-like material. To achieve an improved method that makes it possible to separate lead from coated glass with a very high level of purity and thus allow a complete recycling of said glass, it is suggested according to the invention that the discarded picture tubes are crushed and ground to achieve a finely grained powdered material, that the finely grained material formed is brought into contact with a solution of leaching medium whose task is to solubilise the lead component from the glass material, whereby the components thus separated are sorted from one another via processing by floatation.

Description: Method for separating lead from coated glass The present invention relates to a method for separating lead from coated glass, especially glass from discarded picture tubes, according to the introduction to claim 1. Coated glass is used in a variety of applications within different areas of technology, for example, as a picture tube in television equipment, monitors for computers and surveillance, radar screens, etc. A common factor in all this equipment is that they include several substances that are harmful for the environment, including lead, cadmium, barium, and mercury. A conventional picture tube, or more specifically a Cathode Ray Tube (CRT) includes a picture screen manufactured from glass, a funnel or cone shaped part, and a narrow part that accommodates the electron gun. These parts mainly include SiO , Na O and PbO plus smaller amounts of SrO, BaO, Al 2 O and CaO. It has been calculated that a conventional CRT on average contains approximately 63.5 % screen glass, 24 % cone-shaped glass, 12 % ferrite metals, 0.4 % grating + the electron gun, 0.4 % chemical sealants and 0.04 % fluorescent powder. It has also been shown that each CRT and TV includes between about 0.4 kg and 1 kg lead respectively in the form of lead oxide, which mainly acts as a radiation protection and to stabilise the glass. In addition, the CRT includes barium and several other metals in the fluorescent material and on the inside of the screen, including sulphides of zinc, yttrium and europium or Cds. Several countries regard discarded picture tubes as a hazardous waste that until now have not been recycled but that have instead been crushed to fine particles and then used as filling material or as fuel in special incineration plants. Recently, however, several countries have introduced a ban on using crushed material from picture tubes as it has been shown that heavy metals from the filling material tend to leach into the ground water over time. Incineration of crushed picture tubes is not the sought after solution to the problem either as poisonous gases, so-called dioxides, are formed by the incineration process. Methods known to date for removing the layer and other coated material from pictures tubes have been achieved by different types of processing methods that effect the removal. Compare, for example, DE 42 41 331 that shows and describes a technique comprising water blasting and filtering and DE 41 33 732 that describes a method for removing lead by processing with ultrasound. A procedure for recycling coated glass, for example, picture tubes, where the glass is crushed, separated into different fractions in the form of glass and metal and mixed in a mixing unit for a predetermined period of time during which a liquid circulates through the mixing unit is known from US 5 316 510. According to the procedure, the coating is removed from the glass by the removal effect that is obtained as a result of the tumbling in the mixing unit. The coatings are collected from the circulating liquid by filtering. However, this known method has the disadvantage that it does not allow the release of particles that adhere firmly in the glass or that are in principle enclosed in the glass. The disadvantage of methods known to date for recycling coated glass is that they do not permit a level of purity of the glass such that it can be recycled and be used in a real recycling chain, i.e. to once again be used as a component in glass products such as picture tubes, insulation material, etc. The aim of the present invention is thus to achieve an improved procedure and that makes it possible to remove lead from coated glass with a very high level of purification and thereby also allow the complete recycling of the said glass. These goals are achieved by the procedure according to the present invention having the features and characteristics stated in claim 1. The invention will now be described in greater detail in the form of an example of an embodiment with reference to the enclosed drawings where the single figure shows schematically the principles for carrying out the procedure according to the invention in the form of a block diagram. With reference to the figure, a flow diagram shows schematically the principles for carrying out the procedure according to the invention. Discarded picture tubes are brought from a storage site 1 and are then crushed in a first coarse crushing device 2 to achieve a particle size that is suitable for subsequent sorting in a sorting device 3. This sorting device suitably includes a sifting table in combination with a magnetically acting device for picking up particles of ferromagnetic material from the crushed mass. As picture tubes implode when crushed, it is advantageous from a safety point of view if the they have been ventilated prior to handling and crushing. This ventilation can suitably be achieved by making a hole with a laser or similar. When the coarsely crushed particle material has been separated into a number of its main components, for example, into ferromagnetic and glass-like material respectively, the material composed mainly of glass is transported onwards to a plant 4 for fine crushing and grinding. The particle material is ground down to a very small particle size in the grinding plant 4 and thus forms what is in principle a powder. This step also releases a significant portion of the lead that is accommodated or enclosed in the glass material. At this stage, it should be realised that the particle size chosen at grinding plays a very decisive role for the recovery of lead from the glass at the following separation process step that will be described in more detail later. To also allow the possibility to separate possible remaining particles of ferromagnetic material and/or particles of weakly ferromagnetic material, it is appropriate to locate a further magnetically active sorting device after the grinding plant (not shown in the figure). When the preliminary recycling or sorting processes have been completed, the pulverised glass mass is gathered in a holder 5 for further onwards transport by means of a conveyor 6 for feeding into a feed opening 7 of a screw transporter 8 that in turn supplies a leaching unit 9 with the glass mass. A mixing tank 10 is used to prepare and store a solution of leaching medium whereby alkali hydroxides, i.e. hydroxides and carbonates of sodium and calcium, are suitable used as wetting agents. A preferred leaching medium comprises caustic soda, (sodium hydroxide) and other suitable leaching agents including ammonium carbonates, bicarbonates and sesquicarbonates. To further ensure that the solution of leaching material completely wets the glass mass, the said leaching medium should also contain substances whose task is to reduce the surface tension, for example, synthetic tensides such as alkylaryl sulphonates or ethyl oxidants. The wetting agent is stored in dry form in a container 11 and is fed to the mixing tank via a conveyor 12. Water is led by piping 14 from a water container 13 down to mixing tank 10, whereby the wetting agent is mixed with the water to form a solution by means of a stirrer 15 with rotating blades. By also leading a certain amount of the solution of leaching medium to the feed opening 7 of the screw transporter 8, the friction between the fed mass of glass material and the walls of the screw housing can be reduced and thereby also the power that is needed for driving the screw in the said transporter 8 can be kept relatively low. A heating element 16 is also arranged in the mixing tank 10 to heat the mixture or, more specifically, the solution of leaching medium, to a temperature that is sufficient so that the medium can partly be led to and introduced directly into the feed opening 7 of the screw transporter 8 and partly directly to the leaching unit mentioned above. In leaching unit 9, suitable proportions of regulated concentrations of solid material in the form of a glass mass and a heated solution of leaching medium are mixed to a so-called pulp in which an effective decomposition of the crushed mass occurs. The process of leaching takes place during stirring by means of stirrer 17 with rotating blades and goes on for as long as is needed to separate primarily lead but also other particles from the glass material and to achieve the level of purity desired. The leaching process can also solubilise tightly bound lead particles from the grains of glass, i.e. particles that are more or less located within the glass material and that cannot be removed from the glass by purely mechanical means such as tumbling. It has been shown that an optimal leaching of the crushed mass can be achieved with a liquid solution that comprises 0.5 percent weight or more of caustic soda in water and a temperature of between 50 to 90 °C, preferably 70 °C. However, the caustic soda solution should not have concentrations in excess of 3 percent weight as the cost for the higher concentration of the mixture cannot be motivated by any greater recovery. The temperature in the leaching unit should nevertheless always be below the boiling point. The pulp that mainly consists of the glass mass and solubilised lead particles is led from the leaching unit 9 on to a floatation unit 18 whose task is primarily to sort out lead but also other non-desirable particles from the glass phase part of the pulp. Floatation takes place in what is a per se known manner by utilising the differences in the surface active properties of the component substances where these differences are steered by the additives of different floatation chemicals in the form of so-called raising or sinking reagents so that so that the separated particles are wetted in different ways. In this manner, the air bubbles formed to support the floatation will more easily attach to one of the components, which preferably comprises lead, and this component will be lifted up by the air bubbles to be finally collected at the surface. In the equivalent way, the second component, mainly containing the glass material, will sink and be collected at the bottom of the tank. Foam enhancing agents and so-called collection reagents are added to the floatation process in a known way to achieve the sorting named above and to keep the raised components at the surface. Experience shows that it is also advantageous to maintain a certain viscosity in the floatation unit while the process is being carried out and in this case, a lightly viscous liquid phase is desirable since this facilitates raising and separating the lead particles from the pulp. Maintaining this lightly viscous liquid phase condition is achieved by diluting the pulp pumped out from the leaching unit with a suitable amount of water so that the relationship of the mixture of the solid material - the pulp - and the liquid in the floatation unit 18 of between 50% and 20% is achieved. Floatation unit 18 comprises a container 19 in which a number of partitioning walls 20 are arranged following one another to partially separate sections, so-called cells, that are linked by the flow through the channel that is defined as being between the lower edge of the partitioning walls 20 and the inside of the bottom of the container 10. Just above the bottom in each cell there is a rotating diffusion device that stirs the cell by means of a stirrer 22 equipped with blades and that at the same time also blows out air bubbles through a number of nozzles or mouth pieces in its lower end designated 23. This air is fed to the top of the stirrer 22 of the diffusion device and is led via the rotating axle down to the lower end 23 of the stirrer where the exit openings for the pressurised air are located. These exit openings preferably have a size that is selected so that the bubbles formed have a diameter that varies between 0.015 mm to 0.045 mm, which has been shown to be specially suitable for raising lead particles from the pulp. Due to the current effect that is obtained in the container 19, the glass mass purified from the lead particles will afterwards and following a degree of purification move along the bottom of the container towards an outlet from where the purified glass mass is led on to a drying device 25 via pipe 24. This drying device can be of any suitable type, but is preferably of the cyclone type. The drying device should also be designed so that the liquid that accompanies the glass mass can also be recycled and, after filtration in a filter unit, be once again returned to the separating process by being led to water container 13. The main product obtained from the drying device is purified glass material that can be stored at storage site 27 to await delivery to a subsequent user. The lead-containing foam that has collected and is floating on the surface at the upper part of the floatation device 18 is removed via pipe 28 for later storage and handling. The said removal of foam is suitably performed by equipment that includes a foam removal rake or similar. Other waste products from the returned liquid are also removed from the filter unit 26 via pipe 29 for later storage and handling. The present invention is not restricted to that described above and shown in the drawings but can also be modified and changed in a number of ways within the scope of the concept of the invention as stated in the following claims. For example, it can be considered that the leaching process and the floatation process could be performed in a common step for both processes.

Claims: Claims 1. Method for separating lead from coated glass, especially glass from discarded picture tubes or similar lead-containing glass-like material characterised in that the discarded picture tubes are crushed and ground to achieve a finely grained material, that the finely grained material formed is brought into contact with a solution of leaching medium whose task is to solubilise the lead component from the glass material, whereby the components thus separated are sorted from one another via processing by floatation. 2. Method according to claim lcharacterised in that the leaching and floatation respectively take place in two separate processing steps following one another that are performed in a leaching unit and a floatation unit respectively. 3. Method according to claim lor2characterised in that following the floatation processing, the glass material that has been purified from the lead particles is dried in a subsequent drying step. 4. Method according to claim 3characterised in that the outgoing glass mass liquid is collected during the drying step and that this liquid is returned to the separation process following filtration. 5. Method according to any of the previous claims characterised in that the solution comprising leaching medium and the associated liquid is prepared in a mixing unit that is separate from the leaching unit and/or floatation unit and that the leaching medium is selected from the group of basic alkalis. 6. Method according to claim 5characterised in that the liquid used in the preparation of the leaching medium is water. 7. Method according to any of the previous claims characterised in that caustic soda is used as leaching medium. 8. Method according to claims 5-7 characterised in that the solution of leaching medium formed in the mixing unit is heated by means of a heating element arranged in the said unit. 9. Method according to claim 8characterised in that the solution of leaching medium is heated to a temperature between 50-90 °C, preferably 70 °C.

Inventors: FORSMAN GOESTA, FORSMAN JAN-AAKE, FORSMAN, GOESTA, FORSMAN, JAN-AAKE
Applicants: ECOFLOW SCANDINAVIA AB, FORSMAN GOESTA, FORSMAN JAN AAKE, FORSMAN, GOESTA, FORSMAN, JAN-AAKE
Application year: 1999
Main classification: B03B9
IPC codes: B03B9, B03D1