Glass recycling

Abstract

There is provided a process of recycling glass, and particularly waste domestic glass. A glass powder is partially dissolved in a caustic solution, at a temperature of from 150 to 220°C and at a pressure above atmospheric pressure, to obtain sodium silicate solution and an insoluble powder residue. The insoluble powder residue is separated from the sodium silicate solution and silica is precipitated by acidifying the sodium silicate solution.

Description

GLASS RECYCLINGCROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is claiming priority from U.S. Provisional Application No. 63 / 582,618 filed September 14, 2023, the content of which is hereby incorporated by reference in its entirety.TECHNICAL FIELD

[0002] This disclosure relates to the field of glass recovery and recycling, and in particular methods of recovering glass from caustic solutions.BACKGROUND OF THE ART

[0003] Domestic glass recycling is a multistep process of washing, milling, classifying and decontaminating; resulting in a low-cost glass powder. Depending on the particle size, the classified glass can be used in the glass container industry, in the construction industry, in the glass fiber industry, or it can be used as a paint filler, for example.

[0004] The major constituent of domestic glass waste is SiO2. If the SiO2 can be extracted from the glass waste, products with a higher added value can be produced when recycling the waste glass. The traditional way to extract the silica from domestic waste glass is to dissolve it in a caustic solution at high temperature and high pressure to form a sodium silicate solution and a powder residue with an approximate SiC>2 extraction rate of no more than 50 wt. %. Moreover, this traditional process is not environmentally friendly because of the poor extraction yield. The traditional process includes the steps of mixing sand with sodium carbonate at elevated temperature, approximately 1 ,200 °C, cooling, and after cooling, dissolving the sodium silicate in hot water. Accordingly, this process has a high energy consumption and produces high levels of greenhouse gas (CO2). Therefore, improvements in glass recycling are desired to limit or reduce the disadvantages of the prior art described above.SUMMARY

[0005] In one aspect, there is provided a process of recycling glass, the process includes: partially dissolving glass powder in a caustic solution at a temperature of from 150 to 220 °C and at a pressure above atmospheric pressure, to obtain sodium silicate and an insoluble powderresidue; extracting the insoluble powder residue from the caustic solution to obtain a clean solution; and precipitating the sodium silicate by acidifying the clean solution.

[0006] In some embodiments, the process further comprising recovering sodium silicate from the insoluble powder residue by acid washing, for example with HCI.

[0007] In some embodiments, the process further comprises partially dissolving the insoluble residue powder into a second caustic solution at a temperature of from 150 to 220 °C (or 170 to 190) and at a pressure above atmospheric pressure (e.g. 15 to 150 psi) and further precipitating sodium silicate by acidification from the second caustic solution.

[0008] In some embodiments, the glass powder has an average particle size of from 5 to 30 pm.

[0009] In some embodiments, the caustic solution has a weight ratio of SiC>2 / Na2O of from 1 to 4. In such embodiments, the process may further comprise adding silica to the caustic solution to adjust the weight ratio of SiC>2 / Na2O to be from 1 to 4. In further embodiments, at least a portion of the silica is sourced from the insoluble powder residues.

[0010] In some embodiments, the step of partially dissolving comprises mixing.

[0011] In some embodiments, the extracting step comprises a filtration.

[0012] In some embodiments, the precipitated sodium silicate is a powder having an average particle size of from 15 to 200 pm.

[0013] In some embodiments, the process is performed in an Inconel reactor, and preferably in a closed environment.

[0014] In some embodiments, the insoluble powder residues comprise zeolite and / or calcium silicate.

[0015] In some embodiments, the precipitating step comprises acidifying with an aqueous sulphuric acid solution. In further embodiments, the aqueous sulphuric acid solution has a concentration of sulphuric acid of from 5 wt. % to 17.5 wt. %.

[0016] In some embodiments, the acidifying is performed under heating at a temperature between 60 and 95 °C.

[0017] In some embodiments, the caustic solution has a concentration of NaOH of from 5 to 20 wt. %.

[0018] Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.DETAILED DESCRIPTION

[0019] The present disclosure provides a process of recycling glass (e.g. domestic glass waste), by producing sodium silicate in solution under green and environmentally friendly conditions. The sodium silicate in solution is precipitated by acidification. The precipitated silica obtained presents a higher added value compared to traditional forms of recycling. Indeed, sodium silicate solutions can be used to synthesize different silica materials for example precipitated silica which results from the acidification step. Precipitated silica finds applications in the rubber industry, for example, to improve the resistance property or also as an additive in toothpaste, paint, coatings and the like.

[0020] The present disclosure provides a process of recycling waste glass by partial dissolution of glass powder. First, a partial dissolution of glass powder is performed in an aqueous caustic solution at a temperature of at least 150°C. The partial dissolution can be performed under mixing for example mechanical mixing or magnetic mixing. In some embodiments, the concentration of glass in the caustic solution is of from 100 to 1500 g of glass per kg of water. In some embodiments, the glass powder has an average particle size of from 5 to 30 pm. In some embodiments, the temperature is from 120 to 220 °C, 130 to 220 °C, 140 to 220 °C, 150 to 220 °C, from 155 to 210 °C, from 160 to 200 °C, from 170 to 190 °C or from 175 to 185 °C. In some embodiments, the caustic solution is maintained in a closed environment. In this embodiment, in a closed system, the pressure and temperature are related (e.g. as the temperature increases the pressure also increases). In some embodiments, the dissolution is performed at a pressure above atmospheric pressure, for example a pressure of from 15 to 150 psi, from 15 to 140 psi, from 15 to 130 psi, from 15 to 120 psi, from 15 to 110 psi, from 15 to 100 psi, from 20 to 150 psi, from 30 to 150 psi, from 40 to 150 psi, from 50 to 150 psi, from 20 to 140 psi, from 30 to 130 psi, from 40 to 120 psi, or from 50 to 100 psi . When a closed system is employed, the dissolution may be performed in conditions above the boiling temperature of the caustic solution. In some embodiments, the caustic solution contains NaOH in a concentration of from 5 to 20 wt. % or from 5 to 15 wt. %. In preferred embodiments, the caustic solution has a weight ratio of SiO2 / Na2O offrom 1 to 4, from 1 to 2, from 2 to 4, from 2.5 to 3.5, from 1 .3 to 1 .7, from 1 .35 to 1 .65, from 1 .4 to 1.6 or about 1.5. To maintain the ratio within these aforementioned ranges, an addition of silica can be provided into the caustic solution. The silica can be provided from an external source or can be recovered from the below described steps of the present process.

[0021] The dissolution step results in a formation of a sodium silicate solution and an insoluble powder residue. The extraction of the insoluble ions from the powder residue is then performed by a physical separation method for example filtration. This is followed by acid washing to recover silica. The ions precipitated include pure silica which can be added back to the dissolution step to adjust the SiC>2 / Na2O ratio of the resulting sodium silicate solution or dissolved in a second aqueous caustic solution to form a secondary sodium silicate solution (under the same conditions as the caustic solution). The silicate in both solutions is then precipitated by acidification. Once precipitated the precipitated oligomers can be separated by physical separation methods including but not limited to centrifugation, filtration, and decantation. In some embodiments, the precipitated sodium silicate is a powder having an average particle size of from 15 to 200 pm, from 20 to 200 pm, or from 25 to 150 pm.

[0022] The process herein is preferably conducted in a caustic resistant reactor, preferentially Inconel, to eliminate reactor degradation and external contaminations. As previously described, the reactor can be a closed reactor, for example a hermetically sealed reactor.

[0023] The “powder residue” used herein refers to the insoluble powder obtained after the caustic treatment applied on the glass powder and isolated by filtration. This material is rich in insoluble calcium ions; the residue can present a crystalline structure (zeolite and / or calcium silicate) or be amorphous.

[0024] The acid treatment used herein refers to an acid wash of the residue to extract the insoluble ions and ions impurities from the silica structure by cationic exchange. The resulting powder is a purified amorphous silica powder.

[0025] In some embodiments, the purified amorphous silica powder used herein refers to a powder with SiC>2 content higher than 95 wt. % excluding the water content.

[0026] In one embodiment, the acid for the acid washing step is an inorganic acid, preferably the acid is HCI, and more preferably, the acid is an aqueous solution of HCI 2N. In one example, the washing step is done under reflux during at least 30 min, with a mechanical stirring in aglassware reactor to prevent any erosion and external contamination. The washing can be repeated two times to eliminate all the impurities. After each acid washing step, the purified amorphous silica is washed with deionized water until a neutral pH is reached (e.g. in the range of at least 6.5 to 7.5, 6.7 to 7.3 or 6.9 to 7.2).

[0027] In one embodiment, the acid precipitation is done with an inorganic or organic acid and preferentially with an aqueous sulphuric acid solution in a concentration from 5 wt. % to 17.5 wt. % and preferentially the concentration is 8 wt. %. In one example, first, a quantity of the water glass is placed in the reactor, the pH of the sodium silicate is stabilized at a pH between 7 and 11 preferentially stabilized at a pH comprise between 7.5 and 8.5. Then the solution is heated at a temperature between 60 and 95 °C; preferentially 70 to 90 °C. When the temperature is stable, under mechanical stirring, the second part of the water glass solution is added slowly and simultaneously with the sulphuric acid solution. The pH variation is no more than 0.05. First a trouble, then a suspension of particles is observed. When the totality of the sodium silicate is added and the pH is stable, the solution is aged during one hour at the same temperature. After the aging, the pH of the solution is adjusted to 5 - 5.5 by adding sulphuric acid solution and then cooled down to room temperature. The suspension is filtered and washed two times with deionized water to eliminate the Na2SC>4 salts. The filtration could be done by gravity, centrifugation or preferentially filter press. Then the filter cake could be dried in a rotary oven or slurry of the precipitated silica is preferentially made at a concentration of 5 to 15 wt. % of silica to be spray dried.

[0028] The final sodium silicate solution contains smaller oligomers when compared to oligomer sodium silicate obtained according to traditional methods that operate with a dissolution under atmospheric pressure and a temperature of 105 °C or less. The final product of the present disclosure is a clear orange solution (unlike turbid solutions obtained by traditional methods).EXAMPLESMethods:

[0029] Scanning Electron Microscopy (SEM): SEM images of the precipitated silica were recorded with FEI Quanta-3D-FEG at 3.0 kV without coating or with JEOL 840-A at 15 kV with gold coating.

[0030] Malvern Mastersizer™ analysis: The precipitated silica was dispersed in a volume of water of about 5 mL in an ultrasonic bath for 5 minutes to obtain a well dispersed solution, which was then added into the sonicated bath of Malvern Mastersizer™ 2000 (Hydro 2000S, Model AWA2001) until the obstruction of the signal was about 5 to 8%.

[0031] Specific surface area (BET) and porosity: The surface area and porosity of precipitated silica were characterized with Micrometrics TriStar™ 3000 V4.01 and Micrometrics TriStar™ 3020 V3.02 at 77 K. The collected data were analyzed using the standard Brunauer- Emmett-Teller (BET) to get the surface area, and the pore size was obtained from the maxima of the pore size distribution curve calculated by Barrett-Joyner-Halenda (BJH) method using the adsorption branch of the isotherm.

[0032] X-Ray Fluorescence analysis: Determination of Major and Minor Elements (Si, Al, Fe, Mg, Ca, Na, K, Ti, Mn, P, Cr, V, Zr and Zn) was performed as follows. A portion of sample is mixed with a flux. The mixture is then fused at a temperature of about 1050°C and poured into a mold to form a glass disc. This glass disc will be presented to the X-ray spectrometer. This fusion technique overcomes the inter-element surface, grain size and mineralogy effects that make X- ray spectrometry often difficult. In addition, it allows by the dilution it imposes to reduce the extent of mathematical corrections for software correction of inter-element effects. The extent of the spectrometer allows the acquisition of intensity in kilo counts per second (kcps) for each element of interest. The digital processing program converts these intensity concentrations that are reported as oxides. Samples had the following two size distribution criteria: 98% to 100% passing 106 pm and 85% passing 75 pm.EXAMPLE 1 : Domestic glass recycling: sodium silicate synthesis

[0033] Example 1-1 : in an Inconel reactor (1 L), 222 g of glass powder (with an average particle size of 16 pm), 123 g of NaOH wt. 50 % and 369 g of H2O were added (resulting in a 12.5 wt% NaOH solution). Under mechanical stirring, the solution was heated at 180 °C under 145 psi pressure for 20 minutes. Then the reactor was cooled down until a temperature of 70°C was reached. The solution was then filtered in a Buchner on a paper filter. The total amount of solution isolated in the filtrate represented 1 ,780 g (initial water amount and water used for washing the powder residue): solution D1 . 41 .2 wt. % of the initial amount of glass powder (61 .7 % for SiCh) was dissolved in these conditions. The water glass obtained as a SiC>2 / Na2O wt. ratio of 1 .50. The composition of the residue is presented in table 1.

[0034] The residue powder obtained was washed with a solution of HNO3 4N with a ratio solid / solution of 1 / 10. The residue was charged in a three-bottle necks glass flask, and then the hydrochloric solution was added. Under stirring, the suspension was stirred at room temperature during 1 h, then filtered in a Buchner with a filter paper and washed with deionized water until neutral pH. The powder was dried overnight at 110 °C under vacuum. The composition of the decontaminated silica is presented in table 1 .

[0035] In an Inconel reactor (1 L), 76.9 g of decontaminated silica, 61 .5 g of NaOH wt. 50 % and 184.5 g of H2O were added (resulted in a 12.5 wt. % NaOH solution). Under mechanical stirring, the solution was heated at 180°C under pressure for 20 minutes. Then the reactor was cooled down until a temperature of 70°C and the solution was filtered in a Buchner on paper filter. The total amount of solution isolated in the filtrate represented 1 ,750 g (initial water amount and water used for washing the powder residue): solution D2. 91 wt. % of the initial amount of glass powder (91 % for SiCh) were dissolved in these conditions. The water glass was obtained as a SiC>2 / Na2O wt. ratio of 3.16.Table 1 : Powder composition in wt. % (X-Ray Fluorescence analysis)

[0036] Example 1-2: in an Inconel reactor (150 L), 14.88 kg of glass powder (average particle size is 20 pm), 8.25 kg of NaOH wt. 50 % and 42.795 kg of H2O were added (resulting in a 8.1 wt. % NaOH solution). Under mechanical stirring, the solution was heated at 180 °C under a pressure of 145 psi for 20 minutes. Then the reactor was cooled down until reaching the temperature of 70°C. The solution was then filtered in a polyethylene Buchner funnel (2 ft diameter) on nylon canvas (10 pm of porosity). The cake was washed with 45 L of water. The total amount of solution isolated in the filtrate represented approximately 100 kg: solution D1 . 45.1 wt. % of the initial amount of glass powder (62.1 % for SiCh) were dissolved in these conditions. The water glass obtained as a SiC>2 / Na2O wt. ratio of 1.50. The composition of the residue is presented in the table 2.

[0037] The residue powder obtained was washed with a solution HCI 2N with a ratio solid / solution of 1 / 4. The hydrochloric solution was poured in a glass flask of 22 L, and then the residue was added. Under stirring, the suspension was stirred at room temperature during 1 h, then filtered in a polyethylene Buchner funnel (2 ft diameter) on nylon canvas (10 pm of porosity) and washed with deionized water until neutral pH. The powder was dried overnight at 110 °C under vacuum. The composition of the decontaminated silica is presented in table 2.

[0038] In an Inconel reactor (150 L), approximately 3.95 kg of decontaminated silica, 2.94 kg of NaOH wt. 50 % and 29.6 kg of H2O were added (resulted in a 14.3 wt. % NaOH solution). Under mechanical stirring, the solution was heated at 180 °C under a pressure of 145 psi for 20 minutes. Then the reactor was cooled down until a temperature of 70°C was reached and then the solution was filtered in a Buchner (24 in diameter) on paper filter. The total amount of solution isolated in the filtrate represented 48.75 kg (initial water amount and water used for washing the powder residue): solution D2. 91.6 wt. % of the initial amount of glass powder (94.6 % for SiCh) were dissolved in these conditions. The water glass obtained as a SiC>2 / Na2O wt. ratio of 3.23.Table 2: Powders composition in weight percentage (X-Ray Fluorescence analysis)EXAMPLE 2: Domestic glass recycling: precipitated silica synthesis

[0039] Example 2-1 : The two translucid orange waterglass solutions obtained from Example 1-1 (D1 , D2) were mixed and 3000 g of this new solution (D1+D2) was added in a 5 L glassware 3 necks round bottom reactor. The D1+D2 solution was mechanically stirred at 500 RPM and then heated at 85 °C. When the temperature was stable, the pH was decreased with the help of a peristaltic pump and a H2SO4 46 wt. % solution. First turbidity, then particle suspension was observed when the pH reached 9.5. And a milky white solution was then obtained when the solution’s pH reached 7. The neutralized D1 +D2 solution was stirred for 1 h for the aging step andthe pH was decreased to 5. The solution was filtered on Buchner on 5 pm pore-size nylon membrane and then washed with 4 L of water to eliminate the sodium sulfate salts.

[0040] Example 2-2: The two translucid orange waterglass solutions obtained from Example 1 -2 (D1 , D2) were mixed and 9 kg of this new solution (D1 +D2) was added in a 22 L glassware 3 necks round bottom reactor. The D1 +D2 solution was mechanically stirred at 700 RPM and then heated at 85 °C. When the temperature was stable, the pH was decreased with the help of a peristaltic pump and a H2SO4 46 wt. % solution. First turbidity, then particle suspension was observed when the pH reached 9.5 and a milky white solution was then obtained when the solution’s pH reached 7. The neutralized D1 +D2 solution was stirred for 1 h for the aging step and the pH was decreased to 5. The solution was cooled down to 60 °C and then filtered on a filter press. The resulted cake of precipitated silica was washed 3 times by redispersion with 4 L of water to eliminate the sodium sulfate salts and then pumped again and filtered on the filter press.

[0041] Example 2-3: The two translucid orange waterglass solutions obtained from Example 1 -2 (D1 , D2) were mixed and 106.6 kg of this new solution (D1 +D2) was added in a 150 L stainless steel reactor. The D1 +D2 solution was mechanically stirred at 700 RPM and then heated at 85 °C. When the temperature was stable, the pH was decreased with the help of a peristaltic pump and a H2SO4 46 wt. % solution. First turbidity, then particle suspension was observed when the pH reached 9.5 and a milky white solution was then obtained when the solution’s pH reached 7. The neutralized D1 +D2 solution was stirred for 1 h for the aging step and the pH was decreased to 5. The resulted cake of precipitated silica was washed 3 times by redispersion with 30 L of water to eliminate the sodium sulfate salts and then pumped again and filtered on the filter press.

[0042] The powder compositions were analyzed by X-Ray fluorescence and the results are shown in Table 3. The textural properties of the precipitated silica were also analyzed and the results are presented in Table 4.Table 3: Powder composition in wt%Table 4: Textural properties of the precipitated silica compared to commercial samples

[0043] While the present disclosure has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations and including such departures from the present disclosure as come within known or customary practice within the art and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:1 . A process of recycling glass, the process comprising: partially dissolving glass powder in a caustic solution at a temperature of from 150 to 220 °C and at a pressure above atmospheric pressure, to obtain sodium silicate and an insoluble powder residue; extracting the insoluble powder residue from the caustic solution to obtain a clean solution; and precipitating the sodium silicate by acidifying the clean solution.

2. The process of claim 1 , further comprising recovering sodium silicate from the insoluble powder residue by acid washing.

3. The process of claim 2, wherein the acid washing is performed with HCI.

4. The process of any one of claims 1 to 3, further comprising partially dissolving the insoluble residue powder into a second caustic solution at a temperature of from 150 to 220 °C and at a pressure above atmospheric pressure and further precipitating sodium silicate by acidification from the second caustic solution.

5. The process of any one of claims 1 to 4, wherein the temperature is of from 170 to 190 °C.

6. The process of any one of claims 1 to 5, wherein the pressure is of from 15 to 150 psi.

7. The process of any one of claims 1 to 6, wherein the glass powder has an average particle size of from 5 to 30 pm.

8. The process of any one of claims 1 to 7, wherein the caustic solution has a weight ratio of SiC>2 / Na2O of from 1 to 4.

9. The process of claim 8, further comprising adding silica to the caustic solution to adjust the weight ratio of SiC>2 / Na2O to be from 1 to 4.

10. The process of claim 9, wherein at least a portion of the silica is sourced from the insoluble powder residues.11 . The process of any one of claims 1 to 10, wherein the step of partially dissolving comprises mixing.

12. The process of any one of claims 1 to 11 , wherein the extracting step comprises a filtration.

13. The process of any one of claims 1 to 12, wherein the precipitated sodium silicate is a powder having an average particle size of from 15 to 200 pm.

14. The process of any one of claims 1 to 13, wherein the process is performed in an Inconel reactor.

15. The process of any one of claims 1 to 14, wherein the insoluble powder residues comprise zeolite and / or calcium silicate.

16. The process of any one of claims 1 to 15, wherein the precipitating step comprises acidifying with an aqueous sulphuric acid solution.

17. The process of claim 16, wherein the aqueous sulphuric acid solution has a concentration of sulphuric acid of from 5 wt. % to 17.5 wt. %.

18. The process of any one of claims 1 to 17, wherein the acidifying is performed under heating at a temperature between 60 and 95 °C.

19. The process of any one of claims 1 to 18, wherein the caustic solution has a concentration of NaOH of from 5 to 20 wt. %.

20. The process of any one of claims 1 to 19, wherein the process is performed in a closed environment.

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