Patent Application
20140212347
The present invention relates to a process for recovering lead and/or indium from glass containing PbO and/or indium oxide, primarily from electronic waste material.
Lead is used in radiation shielding glasses in order to absorb gamma radiation and X-rays, e.g. in the cathode ray tubes, CRTs, used in computer screens and television sets, where lowering the exposure of the viewers to soft X-rays is of concern.
Modern CRTs have a front panel made of essentially lead-free glass, there behind a funnel made from leaded glass, and at the far end a neck of highly leaded glass. These CRTs represent an environmental hazard if disposed improperly. In October 2001, the United States Environmental Protection Agency created rules stating that CRTs must be brought to special recycling facilities. In November 2002, the EPA began fining companies that disposed of CRTs through landfills or incineration. Regulatory agencies, local and statewide, monitor the disposal of CRTs and other computer equipment. In Europe, disposal of CRT televisions and monitors is covered by the WEEE Directive 2002/96/EC.
CRT monitors make up about 6% of the electronic waste in Europe, and approximately 450,000 tons of cathode ray tube monitors are disposed of in Europe every year. 70% of all the CRT monitors are dumped in landfills or exported to the foreign countries, since no effective recycling systems are available. As indicated, CRT screens are becoming a bigger problem especially in countries that manufactures these monitors, such as Japan, USA and Taiwan. Many countries handle the CRT waste by sending it to lesser developed countries in exchange for money to avoid regulation protocols. However, this is a temporary solution, as new techniques are being developed to find more effective ways to extract the lead and other hazardous components to be separated from the glass. The rest of said 70% of CRT monitors contain 33,300 tons of lead oxides. Calculated as pure lead, about 31,000 tons of lead is being-dumped in landfills annually in Europe.
The main concern with the dumping of CRT screens is the leaching of lead from the glasses. The leached lead is dangerous both for human beings and environment. Dumping of CRT screens mostly occurs in developing countries. This is a big problem, since it is very common for homeless people, both adults and children, to live in, and around refuse dumps. These people are therefore very exposed to the lead that is leached from screens by the rainwater. Many people who live around refuse dumps therefore become lead poisoned by constant exposure. The most common way for the exposed people to become lead poisoned is that they are drinking water with high content of lead. Lead can also be spread to lands through the air, by mining and by direct discharges in water and lands. It takes a very long time for lead to form harmless compounds in the nature. A big problem is when lead is spread to farms and arable lands. When this happens, people are exposed to lead poisoning from eating the fruits and vegetables that are grown on these lands. Adults and children can also be affected when they eat meat from animals that has grazed on grounds exposed to lead, and also when they eat fish, which is affected by lead that comes from discharges from industries in seas and lakes, leachate from refuse dumps and landfills and outflow from sewages. No safe threshold for lead exposure has been discovered—i.e., there is no known amount of lead that is too small to cause the body harm.
Studies of shredding cathode ray tube glass into cullet (small glass pieces) and reusing them for cathode ray tubes have been made by Association for Electric Home Appliances. Of these studies, a system of extracting a cathode ray tube from a television main body and shredding the cathode ray tube into glass cullet has been developed (see “Electrotechnology”, January, 1997, for example).
A method of recovering glass as cullet is disclosed in, e.g., Japanese Laid-Open Patent Application No. 61-50688. There is also known a method of shredding cathode ray tube glass into cullet (small glass pieces) and reusing them for cathode ray tubes (e.g., Japanese Laid-Open Patent Application No. 9-193762). A method of separating a cathode ray tube into a face plate and funnel in accordance with materials, and shredding them into cullet is disclosed in, e.g., Japanese Laid-Open Patent Application No. 05-185064. Further, a method of separating a cathode ray tube into a face plate and funnel, peeling fluorescent substances and a black mask from the face plate, and recycling the face place is disclosed in Japanese Laid-Open Patent Application No. 7-037509.
To reuse cathode ray tube glass, the glass must be separated into panel glass and lead-containing funnel glass. This is because, if lead is mixed in panel glass by a predetermined amount or more, a browning phenomenon occurs, and the lead-containing glass cannot be reused as a raw material of the panel glass. For this reason, a cathode ray tube is separated into a panel and funnel. For this purpose, there are proposed a method of defining a position to cut a cathode ray tube (Japanese Laid-Open Patent Application No. 9-115449), and a method of melting frit glass, which joins a panel and funnel, thereby separating the panel and funnel (Japanese Laid-Open Patent Application No. 7-45198).
WO 2009/139715 A1 discloses a process for chlorinating ore, slag, mill scale, scrap, dust and other resources containing recoverable metals from the groups 4-6, 8-12, and 14 in the periodic table. It is well known that metal values can be recovered from many sources such as scrap, ores and sea nodules by chlorination. The formed metal chlorides can subsequently be separated and extracted by fractional distillation and condensation, electrolysis of the salt or by hydrometallurgical processing. However, to get a considerably higher reaction rate and yield of valuable metals than what is possible when ferric chloride and/or cupric chloride are used as chlorine donors, aluminum chloride is substituted for said chlorides.
Further, U.S. Pat. No. 4,853,094 discloses a process for the production of metal Me (e.g. Sn) or an alloy containing metal Me from a metal halide MeXn (e.g. Sn Cl4) by electrolysis in a cell having an anode, a liquid metal cathode comprising one or more metals M (e.g. liquid Zn) and a liquid electrolyte comprising a salt melt of one or more alkali metal or alkaline earth metal halides (e.g. LiCl/KCl mixture). The process comprises introducing metal halide MeXn, in which Me represents a metal selected from the groups 2b, 3b (including the lanthanide series and the actinide series), 7b and 8 of the periodic system and Cr, Cu, Au, Ga, Sn, Pb and Bi, X represents halogen and n represents the valence of the metal Me, into the liquid metal cathode and isolating Me or an alloy containing Me from the metal cathode material.
In Journal of Hazardous Materials 161(1109-1113) Issus 2/3 2009 a method of recovering lead through a pyro vacuum process was suggested. However, as far as we know, no method that has been commercially accepted has been proposed that solves the problem of rendering glass from scrapped CRT funnels harmless by recovering lead therefrom.
Another metal oxide that is desirable to recover from glass is Indium. Indium tin oxide (ITO) is one of the most widely used transparent conducting oxides and is e.g. used in flat panel displays such as LCD, LED, OLED, PDP etc, antistatic coatings, in light emitting diodes and in various sources. Indium may also be present as indium doped zinc oxide. The price of indium is high and the supply is low. Hence there is a need of cost effective recycling of indium from glass present in electronic waste material.
A main object of the present invention is to recover metal oxide, in particular lead or indium, from glass containing PbO and/or indium oxide, primarily from electronic waste material.
This object is achieved in accordance with the invention in that the process comprises:
Thereby valuable metals can be recovered from glass as well as making the residue after the process essentially harmless. In regard to lead, it is possible to achieve a lead extraction ratio of more than 95%, which makes the residue after the process essentially harmless.
Preferably the amount of AlCl3 is in the range of 5-20% by weight of the mixture, more preferably 7-15 wt %, most preferably 8-13 wt %.
Preferably, the recovery of at least one of lead and indium from the melt includes:
By selective electro-deposition it is possible to recover most of the oxides present in the glass.
The temperature of the salt melt should preferably not exceed 1000° C. during the dissolution of the cullet, more preferably the temperature should not exceed 900° C., the suitable temperature being of the order of at least 500° C. during the dissolution of the cullet and the electrolysis of the melt. The upper limit is set by the fact that, at higher temperature some chlorides start evaporating from the melt. The lower limit is decided by the liquidus temperature of the salt bath. It is preferable to have the temperature of operation at least 100° C. above the liquidus temperature of the salt bath.
Graphite rods are suitably used for anode and cathode during the electrolysis due to their inertness and low cost. When recovering lead, it is suitable that a pervious cathode diaphragm is provided around the cathode for collecting liquid lead. This is mainly to avoid the contamination of the residue, which is nearly pure silica by liquid lead.
The cathode diaphragm suitably is made from alumina and has a plurality of holes so that the liquid molten salt electrolyte can permeate through these holes while the solid residues can not pass through.
Preferably, a voltage of 2 to 3 V, more preferably 2.3-2.7 V, is used for extracting lead by electrolysis, as it was found to be favorable in the preliminary trials. This is in conformity with cyclic voltameric studies as well with graphite electrodes.
The melt is electrolyzed for a time suitably on the order of 2 to 8 hours, preferably 3-6 hours.
Preferably, the weight ratio between flux and cutlet is on the order of 0.25 up to 1.5, more preferably 0.3-1.0. For lead containing cullet most preferably 0.35-0.45. An increase of flux content and flux./cullet ratio seems not to contribute of extraction ratio of lead oxide since low ratios of flux content and flux/cullet already show very high extraction ratio.
Preferably, the temperature is held for a time on the order of for 4-8 hours during the dissolution step. Then the cullet will be softened and the lead and/or indium value is extracted into the salt melt.
It is possible to vaporize metal chlorides from the melt and condensing them for subsequent recovery of the metals of the condensed chlorides. The metal chlorides from the melt may also be recovered by leaching in water and extracted to recover the metals as hydroxides by a hydrometallurgical method.
After recovering the lead and/or indium and optionally other metals present in the cullet, the chloride salts of the melt may be recycled.
After recovering lead and indium and possibly other metals from glass, a processing residue consisting essentially of Al2O3 and SiO2 remains, which is useful for landfill, building construction, or as a raw material for refractory industry.
Lead containing glass primarily comes from a funnel-shaped part and/or a neck part of a cathode ray tube of a computer screen or a television set.
Alternatively, the at least one of lead and indium can be recovered by a process using a liquid aluminum anode. In this process AlCl3 is generated by an in situ formation during the electrolysis. This process includes the steps of:
The cullet is prepared by crushing glass containing PbO and/or indium oxide. PbO containing glass primarily comes from a funnel-shaped part and/or a neck part of a cathode ray tube of a computer screen or a television set. Indium oxide containing glass primarily comes from flat panel displays coated with indium tin oxide. Preferably, glass containing PbO and glass containing indium oxide are separated, so that the cullet contains lead or indium. However, indium and lead from a mixed cullet can be recovered by selective electrodepositing.
Preferably the salt composition consists of at least two of the salts selected from the group: NaCl, KCl, LiCl, and CaCl2, preferably at least three of the salts selected from the group: NaCl, KCl, LiCl, and CaCl2. Preferably the composition is selected so that the salt composition has a liquidus temperature below 700° C., preferably below 600° C., more preferably below 500° C. For a given combination of salts, the composition is preferably chosen to be within 10% by weight from the lowest eutectic point of the salt combination, more preferably within 5% by weight, most preferably within 1% by weight. However, other contents may be used as long as the liquidus temperature of the salt combination is at least 50° C. lower than the operating temperature during electrolyzing; preferably 100° C. lower than the operating temperature.
In a preferred embodiment the salt composition essentially consists of by weight % of the salt composition, 3-20 NaCl, 30-70 KCl, 20-60 LiCl, preferably 5-15 NaCl, 40-60 KCl, 30-50 LiCl, more preferably 7-12 NaCl, 45-55 KCl, 35-45 LiCl. Such salt composition can provide low liquidus temperatures (eutectic temperature around 350° C.), good electrical conductivity to a comparably low cost.
In an alternative embodiment the salt composition essentially consists of by weight % of the salt composition, 10-50 NaCl, 2-20 KCl, 50-80 CaCl2 preferably 25-35 NaCl, 3-10 KCl, 60-75 CaCl2.
In an another alternative embodiment the salt composition essentially consists of by weight % of the salt composition, 5-20 NaCl, 20-40 LiCl, 40-70 CaCl2 preferably 7-15 NaCl, 25-35 LiCl, 50-60 CaCl2.
In an another alternative embodiment the salt composition essentially consists of by weight % of the salt composition, 35-65 KCl, 20-50 LiCl, 5-20 CaCl2 preferably 45-55 KCl, 30-40 LiCl, 10-15 CaCl2.
To dissolve the cullet, a mixture is preferably provided, which consists of A) 60-95% by weight of the mixture of a chloride salt composition consisting of at least two metal chlorides selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, B) 5-30% by weight of the mixture of AlCl3, and C) optionally 0-10% by weight of the mixture of halides, additional chlorides, sulfides and/or oxides, for instance but not limited to: Cao, Li2O, NaO, MgCl2, BaCl2. In the most preferred embodiment the salt composition consist of NaCl, KCl, and LiCl and having a composition around the lowest eutectic point for the NaCl—KCl—LiCl system. The mixture is heated to form a salt melt. The cullet can be added before heating the mixture or after the salt melt has been formed. This mixture is heated in a refractory container under protective atmosphere, suitably argon, to form a melt. The atmosphere may also be nitrogen. Furthermore, chlorine gas may bed admixed to the nitrogen or argon atmosphere. To dissolve the cullet, the melt is kept at high temperature usually for a time between about 4 and about 8 hours. As a rule, the amount of cullet is preferably such that a weight ratio flux/cullet is between about 0.25 and about 1.5, more preferably 0.30-1.0, most preferably 0.35-0.45. The temperature should be lower than 1000° C., more preferably lower than 900° C. Otherwise lead chloride may evaporate from the melt. For optimal economy the temperature is preferably in the range of 550-700° C. during the dissolution of the cullet and the electrolysis of the melt, more preferably 580-650° C.
For PbO the dissolving reaction is:
3PbO+2AlCl3→3PbCl2+Al2O3 (1a)
A flux/cullet ratio of 0.37 appears to be sufficient to dissolve lead oxide in the salt melt.
For indium the dissolving reaction is:
In2O3+2AlCl3→2InCl3+Al2O3 (1b)
Since the aluminum chloride is difficult to recover after the extraction process, it is desirable that there is no excessive addition of aluminum chloride.
To recover at least one of lead and indium from the melt, various processes that are known per se may be used. It is interesting to note that the salt melt used for extraction can be recycled. The lead and/or indium as well as other metals can be selectively electro-deposited from the salt melt. However, it is also possible to use vaporization of the metal chlorides and condensing them, or leach the salt phase in water and extracting the metals as hydroxides by hydrometallurgy method. The process can be designed to be continuous by combining the two steps. The anode off-gas from a subsequent electrolysis, Cl2, can be reused for accentuating the dissolution of slag/ores. The residue after processing glass consists essentially of Al2O3 and SiO2 and can be used for landfill, building construction or as a raw material for the refractory industry.
Electrolysis using conventional anode/s and cathode/s The electrolysis preferably is carried out in the refractory container that holds the salt melt with the dissolved cullet, and the lead and/or indium and possibly other metal is recovered as a cathode deposit.
In one embodiment lead is recovered from PbO containing glass. Two electrodes of graphite are immersed into the salt melt containing dissolved cullet and connectable to a DC source, which can deliver a DC current at a voltage of 2 to 3 V, preferably 2.3-2.7 V, so that the following reactions'occur with respect to lead.
2Cl−→Cl2(g)+e− (2)
Pb2++2e−→Pb(l) (3)
2Al3++3O2−→Al2O3(s) (4)
This is around 1 volt higher than the theoretical decomposition voltage for PbCl2 to compensate for polarization and other effects. The voltage chosen is low enough to avoid decomposition of AlCl3. The decomposition of AlCl3 in the electrolysis process also need an over-voltage in regards of the theoretical decomposition voltage of AlCl3. In this case, 2.4+1=3.4 V. Thus a voltage lower than 3 V avoids electrolytic decomposition of AlCl3.
Indium can be co-deposited with Sn when the indium oxide is indium tin oxide or thay may be selectively electrodeposited.
Preferably the batch of melt is electrolyzed for 2 to 8 hours. During the electrolysis, the temperature is preferably held above 500° C., more preferably about 600° C.
For collecting liquid lead, a pervious cathode bag may be provided around the cathode. The cathode bag suitably is made from alumina and has a plurality of holes, through which the ions can pass. The holes may be cuts extending in the circumferential direction.
After recovering lead and/or indium and possibly other metals, the chloride salts of the melt may be recycled. Then, a processing residue consisting essentially of Al2O3 and SiO2 remains, which is useful for landfill, building construction, or as a raw material for refractory industry. The lead containing glass processed in accordance with the invention primarily comes from a funnel-shaped part and/or a neck part of a cathode ray tube of a computer screen or a television set.
In an alternative embodiment, an aluminium melt form the anode or a part of the anode, for instance by immersing an electrode, e.g. a graphite electrode, in the aluminium melt and connecting it to positive polarity during electrolysis. Alternatively, the crucible is at least partly made in a conductive material being in contact with the aluminium melt, and connecting the crucible positive polarity during the electrolysis. Thereby, the crucible and the molten aluminium operate as an anode. Of course at least one cathode is still required during electrolysis, e.g. one or more graphite electrode/s submerged in the salt melt voltage
When using an aluminium melt at the bottom of the crucible as the anode or part of the anode, the salt melt and the aluminium are heated to a temperature where both are in liquid phase. To improve viscosity of the salt melt, the temperature of the salt melt is preferably at least 50° C. above the liquidus temperature of the salt melt, more preferably at least 100° C. above the liquidus temperature of the salt melt. The temperature should be at least 660° C. and not more than 1000° C., preferably the temperature is in the range of 700-900° C.
During the electrolysis metals/s from metal chloride/s is deposited at the cathode. At the contact surface between the salt melt and the aluminium melt chloride ions are reacting with aluminium, thereby forming AlCl3. This means that during steady state the salt melt can be wholly or partly self-supporting in regards of AlCl3 and also that emission of chlorine gas is reduced. Lesser amounts of chlorine gas may form even when using an aluminium melt as the anode or part of the anode. This gas may be recovered.
At the cathode metal/s are deposited in solid or liquid state for metal/s with lower melting point than the temperature of the salt melt. For collecting liquid metal, e.g. lead, a pervious cathode bag may be provided around the cathode. The cathode bag suitably is made from alumina and has a plurality of holes, through which the ions can pass. The holes may be cuts extending in the circumferential direction.
The composition of the salt melt is preferably the same as when using a conventional graphite anode/s.
An initiating chloride donor is provided to start the reactions in the salt melt. The initiating chloride donor may be aluminium chloride and/or at least one metal chloride that can be electrolyzed, i.e. so that chloride ions forms AlCl3 at the contact surface between the salt melt and the aluminium melt.
In one embodiment, the initiating chloride donor includes a metal chloride of the same type as provided in the chloride salt composition, e.g. at least one metal chloride selected from the group consisting of chlorides of Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, and Ra.
In a preferred embodiment, the initiating chloride donor includes aluminium chloride added to the mixture before heating it or to the salt melt, said aluminium chloride being added up to 20% by weight of the salt mixture, preferably 1-15% by weight, more preferably 5-10% by weight.
When using aluminium melt as the anode or part of the anode the steps dissolving and recovering by electrolysis are expedited simultaneously, preferably for at least 2 hours.
As the indium and/or lead is deposited, additional cullet can be stepwise or continuously added to the salt melt. The electrolysis and dissolving operation can for instance be performed for 2-8 hours; where after metals deposited at the cathode/s is collected, and the electrolysis can be restarted. To avoid interrupts of the electrolysis, another “clean” electrode can be submerged. Alternatively, the salt melt may have a plurality of electrodes which one after the other is activated as a cathode and while the former is deactivated. Thereby the metals can be selectively deposited at individual electrodes.
The residue after processing contains Al2O3 and other stable oxides such as SiO2.
A sample of CRT glass was supplied by East Japan Recycling Systems Corporation (EJRS). The CRT monitors were taken apart, and some of the parts were recycled. The treatment of CRT monitors by EJRS is described in the following.
The monitors were taken apart by hand, and the CRTs were separated. Printed boards, deflecting yokes, interconnections and speakers were also taken and recycled. The separated CRTs were polished by a brush cleaning equipment, and excrescences on the surface of CRT were removed. Then the panel and funnel were separated by a P/F divider. Since the glass compositions of the panel and funnel are different, they have to be recycled separately. The funnel glass was washed and dry-scrubbed to remove the paint of carbon and iron oxide on the surface of the glass, and then the funnel cullet was produced. This cullet is the raw material of CRT glass.
As indicated above, the glass used in a CRT is of different kinds in the panel, the funnel and the neck of a CRT. The panel contains a barium-strontium glass, the weight of which is about two-thirds of the CRT monitor's total weight. The funnel is made of leaded glass. The weight of it is about one-third of the CRT monitor's total weight. The neck is made out of a high lead content glass, which surrounds the electron gun.
Two different analyses were made of the funnel cullet supplied were made. The result of the analyses is shown in Table 1, where the two different analyses are designated CRT 1 and CRT 2.
0~0.1
The metal chlorides were supplied by Sigma-Aldrich, USA, and the refractory container was an alumina crucible (99.5%, 40 mm outer diameter, 36 mm inner diameter, 60 mm height, from KERANOVA AB, Sweden) having a lid. The chloride salts (8% by weight of NaCL, 53% by weight of KCL and 39% by weight of LiCl) and the cullet were mixed evenly, placed in the crucible and held for a predetermined time in an oven. Argon gas (ICP5.0, AGA gas AB, Sweden) was introduced from the bottom of the furnace as protective gas. Graphite rods (diameter of 6 mm, Lorraine, Paris) were used as electrodes and iron rods (6 mm diameter) as lead wires.
During the dissolution process, the electrodes were held 2-4 cm above the salt melt. After the salt mixture was melted and held for dissolution at a fixed time, the electrodes were immersed in the salt melt, and electrolysis started, supplied by DC power (HP Hewlett 6632A). After electrolysis, the electrodes were taken out from the salt melt, held above the salt melt and cooled down under the protection of argon gas. The sample in the crucible was dissolved in the distilled water completely and filtrated. The residue was prepared for chemical analysis. ICP-AES analysis was applied for the elements Al, Fe, Sr, Zr, Sb, Ba, Zn, Pb, Li and Ti. Atomic absorption analysis was applied for the elements Na, Mg, Ca and K. The deposition products on the cathode were analyzed by Scanning Electron Microscope (SEM) (JSM-840, JEOL) equipped with Energy-Dispersive X-ray Spectrometry analysis (EDS link, Oxford).
Four factors, viz. holding temperature, holding time, flux content (mole ratio to NaCl—KCl—LiCl mixture) and flux/cullet (weight ratio), were investigated. The parameters varied during the tests were temperature (600, 800 and 1000° C.), holding time (4 and 8 hours), flux content (11, 15, 19 and 29 wt %) and flux/cullet (0.37, 0.5, 0.7, 0.9 and 1.5) were applied. The extraction ratio of various metal oxides in the CRT glass in the salt melt is shown in Table 2. In this table, the extraction ratio is defined as the ratio of remained oxides in residue versus the amount of corresponding oxides in the initial CRT glass.
Two methods were used to estimate the amount of extracted oxides. First, the residue after dissolution was filtrated, and then subjected to two chemical analyses, viz. ICP-AES and atomic absorption analysis. It should be mentioned that this extraction ratio is not always equal to the solubility in the salt melt. Some of the oxides can be evaporated as chlorides. Extraction ratio of aluminum and lithium oxide shows a negative value, which means that the amount of the oxide increased. Almost all of the oxides show very high extraction ratio, except iron and antimony oxide. However, iron and antimony oxide show an extraction ratio of approximately 70%. Concerning lead oxide, the ratio is about 50-60% at 800° C., and 91% at 1000° C. These values are lower than that obtained at 600° C. In addition, increases of flux content and flux/cullet ratio appear not to contribute to increasing the extraction ratio of lead oxide, since already low ratios of flux content and flux/cullet show very high extraction ratios. In the present experimental tests, a flux/cullet ratio of 0.37 appears to be sufficient to dissolve lead oxide in the salt melt. This value is the theoretical amount to complete reaction (1) above.
The variation of lead oxide extraction ratio due to different process parameters was investigated. The extraction ratio of lead oxide at 600° C. was approximately 96% and did not depend on the holding time. Concerning variation with temperature, the extraction ratio was about 95, 60 and 91% at 600, 800 and 1000° C., respectively. The ratio seems to be random. However, it can be said that the high temperature did not contribute to a rise of the extraction ratio. Further, high flux content did not give a significant change in the extraction ratio. Similarly, a change in the flux/cullet ratio did not result in any significant change of the extraction ratio of lead oxide that was approximately 95%. Consequently, higher temperature, flux content and flux/CRT ratio, longer holding times are not needed. As mentioned above, already lower values of the parameters give high extraction ratios, and therefore an increase of parameter values does not result in any a rise of the extraction ratio. Based on the results, the optimized process parameters were chosen as-600° C. of temperature, 0.37 of flux/cullet, 4 hours of holding time, and 11% by weight of flux content. In the test at 1000° C. the evaporation of AlCl3 was high. It is therefore preferred to keep the temperature lower than 1000° C., preferably lower than 900° C.
The dissolution test proves that the metal values in CRT glass can be extracted into the salt phase. Under different cell potentials, various metals or alloys can be deposited on the cathode surface. All of the metal chlorides in the melt have a theoretical decomposition voltage. The decomposition voltages of silicon chloride and lead chloride are close to each other, which means that it is expected to be difficult to depose the two separated from each other. However, it is very difficult to dissolve silicon oxide in the salt melt, and thus silicon tetrachloride does not practically exist in the melt. Consequently, it is possible to deposit lead selectively. It should be noted that the electro-deposition of metals can be affected by a number of factors even in pure molten salt system, as for example, over voltage, current density, current efficiency, electro bath conductivity, the nature and surface of the cathode material and distance between electrodes, etc.
In the experimental electrolysis process, 600° C. of temperature, 0.9 of flux/cullet, 4 hours of holding time and 19 wt % of flux content were selected because of the small size of the crucible. Then, the amount of CRT glass was 9.3 g, which contained approximately 1.8 g of lead oxide. If 100% of the lead oxide is dissolved in the salt melt and all of the lead is deposited, approximately 1.7 g of lead is recovered. This amount of lead is the available maximum value for a small crucible.
As demonstrated above, a salt extraction process has been developed for the recovery of lead and other metals from CRT glass. According to the analysis of CRT glass, the glass contains approximately 19% of lead oxide, and approximately 96% of lead oxide was extracted by the dissolution in the salt melt step. The optimized process parameters in the dissolution step were (a) a temperature of 600° C., (b) a holding time of 4 hours, (c) a flux/cullet ratio of 0.37, and (d) a flux content of 11 wt %. Several electrolysis tests were performed with the molten salts and CRT glass. A voltage of 2.3 V was used to electro-deposit lead. A liquid phase of lead was deposited and dropped from the cathode into a cathode bag, where it was collected.
The main concern with the dumping of cathode ray tube (CRT) screens is the leaching of lead from the glasses. Lead is used in radiation shielding glasses in order to absorb gamma radiation and X-rays, e.g. in the cathode ray tubes used in computer screens and television sets, where lowering the exposure of the viewers to soft X-rays is of concern. The leached lead is dangerous both for human beings and environment. The present invention solves the problem of rendering glass from scrapped CRT funnels harmless by recovering lead therefrom. Lead and other metals are present as oxides in the glass and are dissolved in a chloride salt melt, from which they are recovered by electrolysis, for example. The residue after processing, which consists essentially of Al2O3 and SiO2, can be used for landfill, building construction or as a raw material for the refractory industry.
Similarly, indium can be recovered from indium tin oxide coated glass
| Number | Date | Country | Kind |
|---|---|---|---|
| 1150749.8 | Aug 2011 | SE | national |
| 1100604-6 | Aug 2011 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE2012/050885 | 8/17/2012 | WO | 00 | 2/13/2014 |
