Process for reclaiming mirror cullet and production of a powdered glass

BACKGROUND OF THE INVENTION

[0002] A recent survey indicated the manufacturers of glass mirrors in the United States dispose of approximately 40,000 tons of scrap annually. This scrap ends up in landfills. However, the scrap is essentially a good quality soda lime glass which has a mirror coating on one side.

[0003] Mirror coating in the United States typically contains: silver and copper in the metallic state; a cross linked acrylic or similar resin, zinc oxide; barium sulfate; and alumina as fillers. Carbon black is included as a colorant. Normally, a lead compound is included as a corrosion inhibitor. The coating is well engineered in that it resists scratching and penetration extremely well, it resists dissolution into organic solvents and it resists attacks by acids. The mirror coating is the core technology of an excellent product and is responsible for the excellent quality and durability for which the American mirror industry is known.

[0004] If properly tested by TCLP (Toxic Characteristic Leach Procedure, U.S. Environmental Protection Agency), glass cullet with this coating of current production is not measured as a hazardous waste. However, the mirror industry recognizes that the disposal of the cullet in regular landfills entails a risk of liability due to the lead and silver content of the cullet. Moreover, older scrap wastes do test to be hazardous-by-characteristic.

[0005] It is typical for older mirrors to exhibit corrosion. However, current mirrors manufactured in the United States include a leaded corrosion inhibitor. These corrosion resisting mirrors have become the standard of quality for mirrors manufactured in the United States and are, therefore, required for effective competition. Manufacturers have expended considerable research effort in recent years to develop a substitute for lead in the coating. However, most manufacturers still use lead to divert corrosive action. In other words, a leaded corrosion inhibitor is fundamental to the overall quality of the product.

[0006] The market for cullet or scrap soda lime glass has changed. Cullet or scrap soda lime glass has increased in value in recent years. In 1985, it was purchased from dealers at no more than twenty dollars per ton and now commands prices of up to sixty dollars per ton or more. Due to structural changes in melting technology, float producers now use up to thirty percent cullet in their furnaces (formerly, the float producers' use of cullet was limited to less than ten percent in soda lime float batch). Since the value of cullet has increased, additional resources for processing mirror cullet can be deployed if the mirror waste generators are willing to pay a processing fee which is less than the cost of landfill disposal.

[0007] However, before processing the mirror cullet, the mirror coating should be removed. Prior attempts to dissolute the mirror coating by acids and solvents have not been technically or economically feasible.

[0008] The present invention uses loose grain blasting to remove the mirror coating. Loose grain blasting (which utilizes the energy of huge numbers of flying particles moving at high speeds) is the most efficient means for transferring energy from electricity to a surface to be worked. It was introduced as sand blasting in the 1870's after the invention of the compressor (steam powered in those days) for cleaning steel hulls in shipyards. Its use expanded rapidly to industry. A wide variety of abrasive particles were subsequently employed for different purposes. However, the process was not studied in scientific terms for nearly a century until it became an important aspect of aerospace technology. The first papers (Balcar, G. P. & Hanley, W. E. “Usefulness, Effectiveness and Process Technology of Glass Bead Shot Peening”. First World Conference on Aerospace Surface Treatment; Cannes, 1970. Surfaces, Paris. November, 1970. Abrasive Engineering, April, 1971. Metal Treating, June, 1971. Oberflachen Technik, Vol. 50, P. 316. See other work of W. Earl Hanley and Frederick C. Maltby on measurement of speed of cleaning; surface finish and comparison of behavior of different size ranges of abrasives) were published in the 1970's and addressed the efficiency and effectiveness of grain blasting and set forth principles of operation of the process.

SUMMARY OF THE INVENTION

[0009] This invention utilizes abrasive blasting technology to mechanically remove the mirror coating from mirror scrap. The blasting technology produces a cullet having a clean surface, with metal contamination levels which are hardly detectable after processing. The soda lime glass cullet is etched by the blasting action, but is fully useable as a cullet product for remelting into soda lime flat glass, shapes or for spheroidization into soda lime glass beads. The byproduct of the blasting process may be treated to upgrade such byproduct.

[0010] Information which previously existed relating to the variables of particle size range, air pressure or velocity, nozzle angle and nozzle distance was discovered to be useful and/or critical for enhancing various embodiments of this invention and to maximize its efficiency.

[0011] Loose grain blasting uses large volumes of particles to accomplish the cleaning. This produces a by-product of finely pulverized glass or powdered soda lime glass. This by-product is commercially used in a variety of size ranges for a variety of purposes including as a substitute for mineral products as fillers in plastics and paints. The coating bond of soda lime glass can be improved in comparison to minerals due to surface reactivity. (See work of J. R. Ritter and Albert Fuchs on bonding glass to plastic and points.) Special sizing of the particles can be made either by sieves or by air separation as required by specification. The sized particles command prices which are substantially higher than the price of plate cullet. Powdered soda lime glass is also used as an additive in the manufacture of fiberglass insulation. See references from Compagne General de St. Gobain and Schuler Inc. on incorporation of soda lime glass cutlet into fiberglass insulation. This use is a lower value use and does not require rigid sizing of the particles. Other uses or specifications of powdered soda lime glass require low metal content which necessitates either careful selection and special processing or acid treatment. See specifications by Metal improvement Company; Pratt & Whitney Division United Technologies and General Electric Aircraft Engine Division. Glass powders with these characteristics command higher prices particularly when combined with the requirement for a narrow size range.

[0012] Therefore, another object of this invention is to use the abrasive resulting from the mirror cleaning process as a specified powdered glass material for certain applications. The powdered glass is used either “as is”, or in an upgraded condition in which the material is chemically treated to remove contaminating metals and organics.

[0013] The invention results in a multitude of benefits:

[0014] (1) A waste material which is an expense to mirror manufacturers and which creates extended liability for which the reserving of funds for the treatment of landfill sites might be required is, at a reduced cost, made into a product which is introduced to commerce as commodity soda lime cullet.

[0015] (2) The materials which are used to treat the glass become byproducts which may be used as commodity or specialized products.

[0016] (3) The product may be chemically treated to remove contaminants to upgrade the product. The residual waste from this process is sold to lead smelters to remove lead and silver.

[0017] (4) The waste remaining in this phase of the process is minimized while the liability of the generators for their waste may be greatly reduced.

[0018] The various embodiments of the invention(s) generally have or may have: (1) a process for the removal of mirroring coatings from soda lime glass (or from cullet of formulations other than soda lime glass) used in mirror manufacturing by blasting with pulverized soda lime glass or other particulate or loose grain abrasives to produce clean soda lime plate cullet. This blasting is done in a contained space with multiple nozzles blasting of sufficient abrasive to clean the cullet at a specified process speed; (2) a way to produce a soda lime glass powder product comminuted in the process of cleaning the mirroring coatings. Other powder products are created by use of other loose grain abrasives; (3) a way to convey the mirror cullet into the cleaning process which maintains the largest possible cullet shapes for ease of processing; and (4) a process for upgrading the powder product by removing metallic contamination, lead and zinc compounds and organic contamination, all originating from the mirror coating removed by the process described under number 1 above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]
FIG. 1 is an elevational view of the dumping machine and a portion of the conveyor.

[0020]
FIG. 2 is a flow diagram for the blasting portion of the process.

[0021]
FIG. 3 is an elevational view of the blasting process equipment.

[0022]
FIG. 4 is a flow diagram for the blasting machine.

[0023]
FIG. 5 is a flow diagram relating to upgrading the dust resulting from the blasting process.

[0024]
FIG. 6 is a table summarizing data on various abrasives used for blasting.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION(S)

[0025] Referring to FIGS. 1-3, for purposes of removing mirror coatings 10 during the blasting process, it is extremely important to keep the pieces of cullet 12 at the largest feasible size. Commonly, cullet 12 is dumped from collection bins into dump trailers (not shown), dumped again into processing containers (represented at 14) and hauled over-the-road. This causes cullet pieces 12 to break which greatly reduces the size of the cullet pieces 12. One object of the invention is to keep the pieces as large as possible by utilizing gaylord containers (represented at 14) made of cardboard or reusable materials for the essential conveyance of the cullet 12 after it is generated at generators' plant (not shown). Such containers may be positioned at each cullet generating station in generators' plants where the cullet 12 is placed in the containers, coating-side 10 up. These containers are collected and shipped (represented at 14) in box trailers to avoid rain or other wetting. The cullet 12 must then be unloaded (represented at 16). Dumping machine 17 is used to slide the cullet 12 from the gaylord containers and/or a hopper 18 with a hopper extension mechanism 19, preferably in layers, onto a first vibrating conveyor 20 which may, for example, be eight feet long. The device for unloading the containers or dumping machine 17 shown in FIG. 1 tilts and rises to slide the top pieces of mirror cullet 12 onto the vibrating conveyor 20 first, and then tilts and rises further and subsequent layers of mirror cullet slide out and so forth until the container/hopper 18 empties. The dumping machine 17 is shown at a position of maximum extension in FIG. 1. The first vibrating conveyor 20 may move the cullet 12 to a second vibrating conveyor 25 (which may for example be eighteen feet long) which positions the cullet 12 underneath the blasting nozzles 30 of the blasting machine or area 32 such that the coating side 10 of all pieces face the blasting streams of abrasive 34 and stay on the vibrating conveyor 25. This reduces the handling of the cullet 12. Pieces of cullet 12 which are too small to be used with compressed air blasting (e.g. ones which may be moved by the blast) should be removed since they will not be properly treated. One way to remove them is to have these pieces fall through an adjustable opening 40 into a second hopper 42 between the vibrating conveyors 20 and 25 and then move such pieces for alternate processing.

[0026] The particular abrasive 34 used for blasting is then impacted on the mirror coatings 10 with dry compressed air or other means for propelling the abrasive 34 at a desirable pressure or velocity. The abrasive 34 may be powdered soda lime glass or any other particulate, grain or vitreous abrasive, which, in turn, can (depending upon the abrasive) be recycled into, for example, powdered glass or powdered filler products such as Alumaglass, fused aluminum oxide, garnet or crystalline abrasives such as sand or zirconia. The abrasive(s) 34 are placed in a storage device 47. The abrasive 34 may be blasted against to the surface of the cutlet 12 at a pressure of, for example, ninety psi for suction nozzles 30 or sixty psi for a direct pressure system. The distance between the tips of the nozzles 30 and the cullet 12 is preferably six to twelve inches depending upon factors such as nozzle characteristics. The nozzle angle may vary between zero and 180 degrees, and may for example be set at eighty-five degrees. The individual specifications may vary, but testing indicates these parameters are presently preferred.

[0027] Without limitation, the present optimum size range of abrasives 34 are: (1) for soda lime glass US sieve 40-80 (“US Sieve 40-80” refers to particles passing the US Sieve #40 and retained on US Sieve #80.); (2) for Alumaglass FEPA size sixty (“FEPA” refers to the Federation of European Producers of Abrasives. Their specifications for size ranges are now widely used); (3) for fused aluminum oxide FEPA size eighty; and (4) similar size ranges for other abrasive materials, such as, for example, hard or metallic particulate.

[0028] Referring to FIGS. 2-4, the blasting process 32a results in a spent abrasive 34a which may be inspected 33, stored 35 and then used 37 for various purposes as discussed herein. The blasting process 32a is preferably conducted in an area of strong negative pressure which draws the abrasive 34 through a hopper 36 and/or 39 at the bottom of or bordering the blasting area 32, into a first cyclone separator 50, or any other suitable device such as an air wash device. The air wash device/cyclone separator 50 returns the reusable abrasive 34 as active abrasive and draws the fractured abrasives and coating debris into a dust collector 54 or by means of a second cyclone 52 directs this spent material to packaging as products (e.g. plastic filler) or to interim packaging awaiting further processing. To integrate processing of the powders resulting from the blasting the products, the particle separation system can include other cyclones or other particle separating devices to pre-distribute the glass particles into size ranges needed for subsequent marketing or the system can further discriminate between the glass powder or particulate and the coating debris. In the end, only clean air meeting EPA or OSHA standards is emitted from the process. All of the materials are controlled inside the machine until they are packaged, contained or reused.

[0029] The processed cullet may emerge from the blasting area 32, move to a swivel conveyor 38 and then pass to another hopper 39. It should be quality inspected 44, possibly reprocessed, and finished cullet may be stored or loaded for shipment 46.

[0030] Blasting machines 32 are designed to contain all of the residual dust from blasting in either a conventional bag house or a cartridge type dust collector 54 which may contain very fine material to be handled separately, or the dust may be utilized as suggested above. The second cyclone 52 may be used to remove all of the dust from the air stream after the first particle separation for placement in super sacks for delivery to customers as dust product 56 or for subsequent chemical treatment (see FIG. 5 and discussion below).

[0031] The testing used to establish this portion of the invention was carried out according to the protocols in the referenced technical papers. Testing was performed in the laboratory using a machine of smaller size which contained and separated the abrasive 34. Sixty-eight individual test blasts were performed for comparing three abrasives: pulverized soda lime glass; Alumaglass (a manufactured product for loose grain applications); and fused aluminum oxide (a traditional angular fine abrasive). These tests clearly demonstrated that the eighty-five degree nozzle angle produced a sufficient blast more quickly than produced by any other angle. The sizes of abrasives which can be used to complete blasting most quickly were determined to be US sieve-40+80 for the soda lime glass, FEPA size #60 for Alumaglass and FEPA size #80 for aluminum oxide. The size US sieve-40+80 of the powdered soda lime glass is approximately the same as the FEPA size #60.

[0032] Speed tests were conducted both by cleaning a fixed area and calculating the square inches cleaned per second and by cleaning for a fixed time and then measuring the square inches cleaned per second. These tests were all conducted at the eighty-five degree angle and at a fixed nozzle distance of twelve inches. When blasting pressures were varied, it was determined there was a significant difference between cleaning at fifty psi and cleaning at sixty psi, indicating that the blasting velocity of the abrasive is an important variable in cleaning the mirror coatings. The test findings were aggregated into a table 90 which compares the three tested abrasives and summarizes comparative abrasive costs per shift in cleaning mirror cullet coatings annually for cleaning eight thousand tons per shift via direct pressure blasting with a ¼ inch nozzle and ⅛ inch grit stem. The findings appear in FIG. 6 (technical reports on these tests are available from Strategic Materials, Inc. located in Houston, Tex.). The powdered soda lime glass proved to be superior in terms of cost and also in terms of production of powder phase product for which markets already exist. Markets also exist for powdered aluminum oxide at relatively higher prices than the market prices for powdered soda lime glass. It is anticipated that markets can be developed for powdered Alumaglass similar to those for powdered soda lime glass and therefore all three abrasives can be used in the inventive process.

[0033] Referring to FIG. 6 the weight loss tests for calculating erosion were conducted by weighing a sample and then removing the mirror coating from the sample and weighing it thereafter. The general conclusion from the tests was that the soda lime powdered glass was the least erosive. Moreover, if soda lime glass is used as the abrasive, the erosion causes no contamination of the powder products. The impact consumption rate or consumption of the abrasive was measured according to the publications referenced above.

[0034] Additional engineering tests investigated the use of pulverized soda lime glass as an abrasive and included tests of surface cleanliness. These tests verified that sixty psi was the preferred pressure for use on soda lime glass in terms of abrasive consumption. They also demonstrated that the speed of abrasive declines from 2.28 square inches per second for fresh abrasive, to 1.87 after a second cycle and to 1.82 after a third cycle. A lower speed of 1.97 square inches per second resulted from fresh abrasive at the test pressure of seventy psi (the laboratory bench tests were conducted at forty; fifty and sixty psi; it was, therefore, necessary to verify that sixty psi was the peak of the speed curve in square inches per second for this set of tests).

[0035] A cleanliness test was performed in a four percent nitric acid solution in which two samples of glass were separately extracted. One had a visibly clean surface and one had a surface which was approximately ten percent covered by mirror coating. The total amount of silver, copper, lead and zinc metal remaining on the visually clean surface was 0.53 ppm. The total amount remaining on the surface which was ten percent covered by the original mirror coating was 1.72 ppm. Based on these results, if the entire 40,000 tons of cullet generated in the Untied States annually were cleaned to the visibly clean state, it would contain about forty-two pounds of these four metals.

[0036] In a pilot test, scrapped material was cut to shapes of five inches wide and thirty-eight inches long for use in the testing process. A total of 298 of these scrap pieces were cleaned. The machine used suction head nozzles at ninety psi, angled at eighty-five degrees and having a cullet to nozzle distance of six inches. Approximately two square inches per second were cleaned under these conditions. The feed rate was 3.8 pounds per minute for ⅜ inch nozzles with {fraction (3/16)} inch diameter air jets. A production machine may also employ ½ inch nozzles and ¼ inch air jets. The blast pressure of ninety psi used during pilot testing was the effective equivalent of sixty psi used in direct pressure equipment. (It is generally believed in loose grain abrasive engineering that 1.5 times the psi of direct pressure will produce approximately the same velocity of particles in suction head nozzles. This is borne out by the speed-of-cleaning observed in the pilot test.). The “suction” design is preferred to the “direct pressure” design of equipment as the suction equipment is capable of blasting continuously while direct pressure is not, without special engineering. In the pilot testing, the estimated cost based on the bench testing was verified at the pilot test feed rate of 3.8 pounds per minute. The feed rate of the ½ inch nozzles is assumed to be approximately ten pounds per minute. Approximately ten percent of the abrasive fed through the nozzles was accumulated in the dust collector during the pilot test indicating a realistic consumption of ten percent per cycle. Pilot and production consumption is normally less than that observed in the laboratory.

[0037] The vibrating tables 20 and 25 which will feed the mirror cullet to the blasting nozzles will have a doctor opening 40 and/or a manual check 49 may be performed to remove pieces of cullet which are too small to be processed in the blasting unit 32. A device such as a metering valve 69 can be made responsive to the check 49 to adjust parameters for the abrasive 34. As depicted by reference numbers 48, 48a and 48b the small removed pieces may be crushed and used as abrasive or alternatively may be cleaned in a tumble belt blasting machine represented by reference number 66 (which costs more than blasting on a flat vibrating table). The blasting of both sides of the cullet is required in tumble blasting. The cleaned product may then either used a finished cullet 68 or quality checked 44 and may be packed for shipping 46 or returned for processing to devices 17, 20. In the case that crushing/pulverizing 48b results in fines too small to be used as an abrasive, the too small fines may be separated by sifting 60, removed for further separation 62 and packaged/contained for further treatment or use as a dust product 64.

[0038] A quality inspection 44 should follow the blast treatment. The inspection may be visual. Cullet having the mirror coating removed as depicted by reference number 46 is packaged in super sacks, Roura cans or transferred directly to a dump truck to be carried to customers. The cullet may also be stored as it is no longer a waste and it is not classified as hazardous.

[0039] The dust resulting from the blasting treatment may be used as a byproduct with no further processing required. It is anticipated this byproduct will be a low priced product. However, referring to FIG. 5, another aspect of this invention provides for upgrading this spent abrasive dust material 37, 56, 64 to a much higher value glass powder. The upgrading is carried out in stages of acid extraction 70; removal of a portion of the silver 72; solvent extraction 74, 76, 78 of filtration sediment 80; and floatation 82 and decanting 84 of organics as shown and described in the flow chart in FIG. 5. The upgrading process may also include phases of drying the solute 78, 86 after the extraction steps. A material resulting from this phase can be sold to lead smelters.

[0040] Experiments were conducted on mirrors having a typical coating. The constituents of the coating were measured as follows (the numbers are approximate):

[0041] Constituents of the Mirror Coating-MG./SQ. FT. and Percent by Weiht

1ProjectedPercentMg./Sq. Ft.By WeightSilver700.91Copper200.26Organics3.80550.65Inorganic Fillers3.66846.53Lead Compounds1271.65TOTAL7690100

[0042] After the blasting media is added, the mirror coating becomes approximately five percent of the total material as follows:

[0043] Contaminants in Dust/Collector Dust

2Mg.Percent/WeightBlasting Media (soda lime glass)150,61095.14Silver700.044Copper200.013Lead Compounds1270.079Inorganic Fillers3.6683.26Organics352.46TOTAL158.300100

[0044] Testing indicated that nitric acid is the most desirable extraction means and that a two percent solution is effective. Extraction is carried out with a quantity of acid 88 (see FIG. 5) which is greater than that which is stoichiometrically necessary at a level of four times the volume of the dust. The bulk density of the dust was measured to be 1.05 grams per cubic centimeter. The volume increase is forty-six percent when equal volumes of the dust and a two percent nitric acid solution are merged.

[0045] The acids were also tested to determine whether they can be reused. It was found the acids are buffered by the action of sodium ions coming off the surface of the dust or glass powder. This reaction is, however, slower than the reaction of the dissolution of the metals into the solution thus the metals are dissolved or reacted before the solution is buffered. The experiments indicated virtually all of the silver and copper react into the acid solution with virtually all of the lead and zinc. After extraction, the remaining sediment was washed and a TCLP was taken. The TCLP indicated no more than 0.5 parts per million of lead and silver leached with 2.5 ppm of barium and 1.4 ppm of zinc leached into the TCLP extractor solution. In addition to the soda lime glass, the sediment contains alumina, barium sulfate, magnesium silicate or magnesium oxide or other silicates of the identified metals.

[0046] The acids and soluble compounds are separated from the residual glass sediment and inorganic fillers and organic binders in a filter press or process 81. The fluid filtrate is then joined with other wastes later in the process which contain similar metals or metal salts. The mixture may be concentrated 83 by reverse osmosis or evaporation and spray dried 86 to produce a commercial grade powder containing the lead, copper, silver and zinc compounds 85. Alternatively, the spent acid can be sent to reverse osmosis to reclaim reusable acid 87 which can be blended with new acid solution to be used in the extracting process 70.

[0047] After filtering 81, remaining sludge may be processed through floatation 82 to remove organics. Floatation may be carried out in a super saturated solution of potassium carbonate. The sludge is mixed with the carbonate and introduced into a floatation cell for the separation. The potassium carbonate is removed with the floatants by decantation 84. The resulting sediment is water washed, filtered 80 and solvent dried 78 to avoid hydrolysis of the sodium ions on the surface of the glass which can cause caking of the powder. The solvent is condensed and recycled 76. The rinse water is sewerable. The sediment depicted as reference number 79 is then packaged for sale or directed for further processing through traditional sieving or air particle size separation. The silver may be removed from the acid solution by plating it onto silver anodes prior to spray drying. The silver may then be sold. The prospective value of the spray dried material containing silver versus that with the silver removed helps determine whether the silver should be removed.

[0048] Various changes can be made to the embodiments described without departing from the spirit of the invention as claimed.

Process for reclaiming mirror cullet and production of a powdered glass

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