Patent Description:
Sodium silicate, with a global market size estimated at USD <NUM> billion and production capacity of <NUM> million tonnes, is one of the most widely used chemicals in both commercial and domestic settings. A combination of having no known ecologically problematic properties and usefulness as a cost-effective binder, pH adjustor, cleaning agent and feedstock for precipitated silica has seen sodium silicate become an integral part of many products including: detergents, concrete hardeners, zeolites, geopolymer concrete, tyres, ceramics, paper and cardboard.

Commonly referred to as waterglass, sodium silicates are manufactured commercially by combining crystalline silica, usually in the form of high purity quartz, and Sodium Oxide (Na<NUM>O). Na<NUM>O reduces the melting temperature of crystalline silica from <NUM> to <NUM> and the resulting glass product is water soluble. Alternatively, liquid sodium silicate can be manufactured via the direct dissolution of crystalline silica in sodium hydroxide under conditions of high temperature and pressure.

Sodium silicates are typically described first by the molar ratio of Silicate: Sodium oxide (SiO<NUM>:Na<NUM>O), with a <NUM>:<NUM> ratio product commonly used in detergent and zeolite production. The hydrothermal route is an economic alternative for production of <<NUM>. 5SiO<NUM>: 1Na<NUM>O sodium silicates.

Estimates of energy requirements of the two production routes for SiO<NUM>:Na<NUM>O products of <NUM>:<NUM> (furnace route) and <NUM>:<NUM> (hydrothermal route) are comparable, with <NUM> GJ and <NUM> GJ per tonne of dry solids respectively. The combination of this energy requirement and the calcination process undergone by parent materials results in a substantial estimated carbon footprint of <NUM> CO<NUM>-e/kg; an Australian figure which includes transportation but not energy expended in the extraction of raw materials.

Soda lime glass (also called soda lime silica glass) accounts for about <NUM>% to <NUM>% of manufactured glass worldwide. Soda lime glass is normally manufactured by mixing silica (typically quartz), sodium carbonate and a calcium material such as lime (calcium hydroxide) and/or calcium carbonate, and a small amount of alumina in a blast furnace and heating to temperatures of greater than <NUM>.

Two categories of soda-lime glass, plate and container, account for <NUM>-<NUM>% of global production. Energy requirements for soda-lime glass vary between <NUM>-<NUM> GJ per ton. The resulting full life cycle footprints of container and plate glasses are estimated to be <NUM>-<NUM> (US) kg CO2-e/kg and <NUM> CO<NUM>-e/kg (EU) respectively.

In theory, glass is infinitely recyclable. However, the reality of recycling glass from solid waste streams, with variable chemical composition of different glass types and colour sorting of glass only possible on particles ><NUM>, shows a very different picture. Of ~<NUM> million tonnes of container glass consumed in Australia in <NUM>, only <NUM>% was recycled; leaving <NUM>,<NUM> tonnes of glass to be stockpiled or sent to landfill. Waste glass represents an estimated <NUM>% of the solid waste stream in high-income countries and the diversion of waste glass to landfill is an issue of growing concern across the world.

It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.

The present invention is directed to a process for the alkaline digestion of soda-lime glass, which may at least partially overcome at least one of the abovementioned disadvantages or provide the consumer with a useful or commercial choice.

With the foregoing in view, the present invention, resides broadly in a method for the alkaline digestion of soda-lime glass, the method comprising:.

In one embodiment, the hydroxide solution comprises an alkali metal hydroxide solution. The alkali metal hydroxide solution may comprise a solution of sodium hydroxide, a solution of potassium hydroxide, or a solution comprising sodium hydroxide and potassium hydroxide. For convenience, hereinafter in this specification, the hydroxide solution will be described as an alkali metal hydroxide solution.

In one embodiment, the mixture formed in step (a) comprises from <NUM> to <NUM> glass per litre H<NUM>O <NUM> to <NUM> glass per litre H<NUM>O, or from <NUM> to <NUM> glass/L H<NUM>O, or from <NUM> to <NUM> glass/L H<NUM>O, or from <NUM> to <NUM> glass/L H<NUM>O, from <NUM> to <NUM> glass/L H<NUM>O, or from <NUM> to <NUM> glass/L H<NUM>O, or from <NUM> to <NUM> glass/L H<NUM>O, or from <NUM> to <NUM> glass/L H<NUM>O, or from <NUM> to <NUM> glass/L H<NUM>O, or from <NUM> to <NUM> glass/L H<NUM>O, or from <NUM> to <NUM> glass/L H<NUM>O of glass, or from <NUM> to <NUM> glass/L H<NUM>O, or from <NUM> to <NUM> glass/L H<NUM>O, or from <NUM> to <NUM> glass/L H<NUM>O, or from <NUM> to <NUM>/L of glass, or from <NUM> to <NUM>/L of glass. Specific examples of the process of the present invention utilised <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> glass/L H<NUM>O in the mixture.

The soda lime glass that is present in the mixture in step (a) may be provided in particulate form. In some embodiments, the glass is provided in the form of glass particles having a size distribution with a maximum particle size of <NUM>, or a maximum particle size of <NUM>, or a maximum particle size of <NUM>, or a maximum particle size of <NUM>, or a maximum particle size of <NUM>, or a maximum particle size of <NUM>. In some embodiments, the glass is provided wherein all of the glass particles are sized less than <NUM>, or sized less than <NUM>, or sized less than <NUM>.

In some embodiments, the soda lime glass comprises waste glass or recycled glass. In these embodiments, it is especially useful that the recycled glass or waste glass has a maximum particle size of <NUM> or less as this represents a fraction which cannot currently be easily separated using optical means and therefore is commonly treated as a waste product.

In some embodiments, the soda lime glass comprises waste glass or recycled glass having a particle size of less than <NUM>.

Without wishing to be bound by theory, it is anticipated that reaction kinetics could be improved if the soda lime glass that is added to the mixture has a smaller particle size.

The alkaline metal hydroxide solution may comprise sodium hydroxide, potassium hydroxide, or a mixture of sodium hydroxide and potassium hydroxide. Other alkaline metal hydroxides may also be used.

In some embodiments, the alkaline metal hydroxide solution may have a hydroxide concentration of from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>. In some embodiments, the alkali metal hydroxide solution may have a concentration within the range of from <NUM> to <NUM>. Concentrations approaching the solubility limit of hydroxides (~<NUM>) could also be used but may limit the achievable SiO2:Na2O ratio of the resultant solution.

The mixture of glass and alkali metal hydroxide formed in step (a) results in the digestion of the glass. The digestion step may take place as a batch, semi-continuous, or continuous operation. In some embodiments, the residence time in the digestion step is greater than <NUM> hour, or from <NUM> hour to <NUM> hours, or from <NUM> hour to <NUM> hours, or from <NUM> hour to <NUM> hours, or from <NUM> hour to <NUM> hours, or from <NUM> hour to <NUM> hours, or from <NUM> hour to <NUM> hours, or from one hour to <NUM> hours. In some embodiments, the residence time is less than <NUM> hours.

The digestion step suitably takes place at atmospheric pressure.

The digestion step is conducted at an elevated temperature, or at a temperature of from <NUM> to the boiling point of the mixture, or at a temperature of from <NUM> to <NUM>, or at a temperature of from <NUM> to <NUM>, or at a temperature of from <NUM> to <NUM>, or at a temperature of from <NUM> to <NUM>, or at a temperature of from <NUM> to <NUM>, or at a temperature of from <NUM> to <NUM>, or at a temperature of from <NUM> to <NUM>, or at a temperature of from <NUM> to <NUM>, or at a temperature of from <NUM> to <NUM>. Under ideal conditions the reaction takes place at between <NUM>°-<NUM>; balancing a maximised dissolution rate and process energy requirements.

In some embodiments, the mixture is stirred or agitated during digestion. The mixture may be stirred using an impeller or a stirrer.

In some embodiments, the mixture in step (a) is controlled such that the addition of water, soda-lime glass and alkali metal hydroxide occurs at stoichiometric ratio is consistent with the desired SiO<NUM>:M<NUM>O ratio in the sodium silicate solution.

Specifically, in some embodiments the molar ratio of SiO<NUM>:Na<NUM>O, considering both glass and alkali metal hydroxide, is between <NUM>:<NUM> and <NUM>:<NUM> with the total molar concentration of SiO2 + Na2O being ><NUM> L-<NUM> H<NUM>O.

Digesting the glass also forms a solid material. The solid material comprises a calcium-silicate-hydrate (C-S-H). Without wishing to be bound by theory, it is believed that the calcium-silicate-hydrate precipitates during alkaline digestions of soda-lime glass.

Calcium-silicate-hydrate is a binding material that can be found in the hydration of cement. Calcium-silicate-hydrate has a pozzolanic potential and the calcium-silicate-hydrate may have some commercial value. Accordingly, the process comprises the step of separating the aqueous sodium alkali silicate from the calcium-silicate-hydrate. The process may further comprise recovering or storing the aqueous sodium alkali silicate and recovering or storing the calcium-silicate-hydrate.

In some embodiments, the step of separating the aqueous sodium silicate solids comprises centrifuging, gravity separation or filtration. Other solid/liquid separation steps known to the person skilled in the art may also be used.

In some embodiments, the solid/liquid separation step may take place at elevated temperatures. The viscosity of the aqueous sodium silicate is likely to be lower at elevated temperatures therefore enhanced separation of the solid and liquid phase is to be expected. In some embodiments, the solid/liquid separation step is operated at a temperature of <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>, or from <NUM> to <NUM>.

In some embodiments, the solid/liquid separation step may further comprise adding additional water to enhance separation. It will be appreciated that adding additional water at this stage affects the concentration of sodium silicate in solution and may also affect the ratio of SiO<NUM>:M<NUM>O in solution.

The present inventor has also found that the solid material recovered from the digestion of the soda lime glass can be further treated to recover silica.

Accordingly, the present invention provides a process for treating a calcium-silicate-hydrate ("CSH") recovered from the alkali digestion of soda lime glass comprising the steps of mixing the CSH with an acid to thereby dissolve soluble metals from the CSH and separating a liquid phase from a solid phase, the solid phase comprising SiO<NUM>.

In one embodiment, the solid phase comprises SiO<NUM> gel.

In one embodiment, the acid is an inorganic acid. In one embodiment, the acid includes Cl- ions or NO<NUM>- ions. Acids containing these anions result in uniform solubility of their derivative salts. In one embodiment, the acid comprises hydrochloric acid or nitric acid.

In one embodiment, the acid is added in excess of the amount of soluble metals present in the CSH. This assists in ensuring that complete dissolution of the soluble metals present in the CSH is likely to occur.

In one embodiment, the CSH is suspended in water or solution prior to or during contact with the acid.

In one embodiment, the solid phase is washed following separation of the solid phase from the liquid phase. The solid phase may be washed with water, or may be washed with dilute acid, followed by washing with water.

Without wanting to be bound by theory, it is believed the reaction proceeds as follows:.

The liquid phase separated from the solid phase in the process of the present invention comprises a leachate containing dissolved metals. Following separation of the leachate from the solid phase, the leachate may be treated to precipitate metal compounds and optionally to separate the metal compounds from the liquid phase.

In one embodiment, metal compounds are precipitated by increasing the pH of the leachate. This may occur by adding an alkali material, such as an alkaline metal hydroxide or an alkaline earth metal hydroxide material, or by adding a basic material such as lime, dolomite or magnesia.

In a further embodiment, the precipitation step may be conducted as a multi-part precipitation in order to separate soluble metals and increase purity of final gel precipitate. In one embodiment, the first step of the precipitation process involves addition of acid to adjust the pH to between <NUM> and <NUM>, or at a pH between <NUM> and <NUM> or, at a pH between <NUM> and <NUM>. Ideally the pH is adjusted to between <NUM> and <NUM> to exhaust the buffering capacity of the CSH and facilitate its immediate and complete dissolution, which is desirable for further separation.

In a further embodiment, the liquid phase can be removed from the silica gel solid by centrifugation and the dissolved metal compounds precipitated by the addition of alkaline compounds. In a further embodiment, the purity of the silica gel precipitate can be improved by leaching the iron salts from the silica gel by using acid to decrease pH to <<NUM> and subsequently washing the solid silica in a dilute acid solution.

There are a number of processes well known to person skilled in the art to cause precipitation of iron containing compounds and other metal containing compounds. All of these processes that are known to the person skilled in the art may be used in this aspect of the present invention.

Further separation of dissolved metals (Mg, Ca, Fe and Al) can be achieved via acid-base reactions well-known in the art such as the separation of Mg and Ca by addition of sulphuric or phosphoric acid which results in the formation of insoluble CaSO<NUM>/Ca<NUM>(PO<NUM>)<NUM> and soluble Mg salts.

In one embodiment of the present invention, the process comprises:.

In some embodiments, the solid material obtained after digestion may also include undigested glass.

In some embodiments of the present invention, the present invention provides a method for the alkaline digestion of soda-lime glass, the method comprising:.

In this embodiment, step (a), which involves digesting the glass, also forms a solid material wherein the solid material comprises a calcium-silicate-hydrate (CSH) and the solid fraction separated in step (b) comprises calcium-silicate-hydrate, and the CSH is treated with an acid to dissolve soluble metals from the CSH and separating a liquid phase from a solid-phase, the solid phase comprising SiO<NUM>. It will be appreciated that the solid material in this embodiment will include undigested glass and CSH. In some embodiments, the method may further comprise separating the undigested glass from the CSH prior to contacting the CSH with the acid and at least part of the undigested glass is returned to step (a). The undigested glass and the CSH may be separated based on density and size differences between the particulate CSH and the undigested glass.

At least some of the SiO<NUM> produced from CSH is returned to step (a). In some embodiments, the SiO<NUM> also contains undigested glass and some or all of the mixture of SiO<NUM> and undigested glass is returned to step (a).

The SiO<NUM> may be in the form of a silica gel.

Recycling of undigested glass and/or SiO<NUM> back to step (a) has the potential to enhance sodium silicate recovery in the method of the present invention.

At least some of the SiO2 is returned to step (a). In some embodiments, the SiO2 may also contain undigested glass and the mixture of SiO2 and undigested glass may be returned to step (a).

Various embodiments of the invention will be described with reference to the following drawings, in which:.

Two sets of experiments investigating the reaction kinetics of alkaline digestion of waste soda-lime glass at standard atmospheric pressure. In the first set of experiments several commercially relevant sodium silicate solutions were obtained with dissolved SiO<NUM> equivalent ranging from <NUM>-><NUM>-<NUM> L-<NUM> and Si<NUM>O:Na<NUM>O+K<NUM>O ratios from <NUM>-><NUM>. The highest accumulation rate of <NUM> SiO<NUM> L-<NUM> hr-<NUM> was obtained between <NUM>-<NUM> hours with <NUM> mixed waste glass (<<NUM>) L-<NUM> stirred continuously in <NUM> NaOH at <NUM>. On average (n=<NUM>) <NUM>%±<NUM>% (<NUM>%CI) of dissolved metal oxides in final solutions were Na<NUM>O, K<NUM>O, or SiO<NUM>. Details of these experiments are given in the examples below.

The second set of experiments tested the influence of temperature (<NUM>, <NUM> & <NUM>) as well concentration of glass (<NUM>, <NUM>, <NUM> & <NUM> L-<NUM> H<NUM>O) and alkali (<NUM>, <NUM> & <NUM> NaOH) on reaction kinetics. Digestions were conducted in triplicate before destructive sampling to determine the recoverability of starting solids in either the liquid (sodium silicate) or solid (CSH) at <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM> hours. As before, accumulation of silicon in the liquid phase increased concurrently with increasing temperature, concentration of glass and alkali. The highest concentration (<NUM> L-<NUM>) of silicon was seen in the <NUM> L-<NUM> H<NUM>O/<NUM>/<NUM> NaOH treatment after <NUM> hours. Recovery of solids in the liquid sodium silicate phase increase with digestion time for all treatments and ranged from <NUM>-<NUM>%.

Mixed consumer waste glass (<<NUM>) was sourced from Owens-Illinois (O-I) International cullet recycling facility in Crestmead, Australia. The chemical analysis of the cleaned glass fraction is shown in Table <NUM> below. Glass was pre-washed in deionised water by <NUM>) submerging the glass in water; <NUM>) stirring the solution to suspend the glass particles; <NUM>) allowing the solution to settle and; <NUM>) removing excess liquid and organic fraction which had settled on the surface. This was repeated until minimal organic fraction was present, and the washed glass was then oven-dried at <NUM>.

Table <NUM> shows the chemical analysis and d(<NUM>) of particle size distribution of the waste glass used in the experiments. Values in Table <NUM> are the average of <NUM> samples taken after washing pre-treatment with +/- <NUM>% confidence interval. <FIG> shows the weight percentage of glass in each particle size range.

Reactors were manufactured by enclosing <NUM> stainless steel beakers (<NUM> grade) with EVA foam lids. Agitation was maintained by <NUM> x <NUM> horizontal stainless steel (<NUM> grade) stirrer blades attached to an overhead stirrer (IKA, China) by a <NUM> stainless steel shaft. A polypropylene sleeve was fitted into the foam lid to reduce friction between it and the stirrer shaft while minimising water loss from the reactor. The reactors were placed in a water bath with an immersion heater/circulator (Ratek, Australia) to control the temperature. Additional water was added to the bath every <NUM>-<NUM> hours to ensure at least <NUM> of the reactor remained immersed for the duration of the experiment.

Table <NUM> summarises the reaction conditions for examples <NUM>-<NUM>.

Table <NUM> summarises the reaction conditions for extraction runs <NUM>-<NUM> in the second set of experiments.

Anhydrous NaOH or KOH (><NUM>%) was dissolved with gentle stirring at room temperature in the stainless-steel beaker with <NUM> of Milli-Q water. The water bath was then raised to operating temperature and glass was added to the solution with the stirrers operating at <NUM> rpm.

In the first set of experiments, reactors were run under the stated conditions until, in some cases, it was no longer possible to obtain a supernatant post centrifugation. At this point, <NUM>-<NUM> of Milli-Q water was added to the reactor to continue the experiment. Extractions marked with an *, all in the <NUM> treatment, required early interventions due to equipment failure (mostly faulty lids allowing water to evaporate). Where possible these experiments were repeated and, unless otherwise specified, unadulterated experiments are reported on here. Faulty runs are included as they provided interesting, otherwise unrealised, insights into reaction kinetics.

In the second set of experiments, digestions were run in triplicate up to and including the sampling event at <NUM> hours. At this point, and after <NUM> and <NUM> hours, one of the reactors was destructively harvested by centrifuging the entire contents of the reactor for <NUM> x <NUM> at <NUM>/<NUM> rpm and collecting the supernatant. In the case of the treatments where the digester was operating at <NUM>, the solids fraction was then resuspended in the empty reactor with <NUM> of Milli-Q water and stirred at <NUM> rpm for <NUM>. The slurry was then centrifuged for <NUM> x <NUM> at <NUM>/<NUM> rpm and the supernatant and solid fractions collected and stored. Density of silicate solutions were determined gravimetrically by weighing <NUM> of solution after standing for <NUM> hours at room temperature. Recovery of starting solids in liquid phase was calculated as the % of initial solids present in the initial centrate and the centrate from the single 'wash' in <NUM> of water.

In the first set of experiments, samples were taken every <NUM>-<NUM> hours depending upon temperature and apparent speed of dissolution. An aliquot of <NUM>-<NUM> was pipetted from the reactor into a <NUM> Falcon tube. The <NUM> Falcon tube was placed in a water bath at the same temperature as the reactor for <NUM> before being removed and centrifuged immediately for <NUM> minutes at <NUM>/<NUM> rpm. <NUM> of supernatant was then pipetted into a <NUM> Eppendorf tube and kept for analysis.

In the second set of digestions, <NUM>-<NUM> aliquots were taken after approximately <NUM>, <NUM><NUM>, <NUM>, <NUM> and <NUM> hours, with the exact time recorded for each sampling event, and then processed as above.

All samples were analysed for soluble cation concentrations (Si, Na, K, Al, B, Ca, Cr, Cu, Fe, Mg, S, Zn) using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) (Perkin Elmer Optima 7300DV, Waltham, MA, USA). Solid samples and liquid samples with high viscosity (gel) were dissolved using microwave digestion with HF acid and measured using ICP-OES.

The purity of the sample was determined by the equation:.

Where Purity% is weight of Na<NUM>O+K<NUM>O+SiO<NUM> (g L-<NUM>) as a percentage of the sum of all metal oxide weights and MO is the sum of oxides of Si, Na, K, Al, B, Ca, Cr, Cu, Fe, Mg, S and Zn.

Calculated by determining the amount of each element present in the liquid phase (concentration x volume) and converting to solid equivalents (e.g. calculated on a Na<NUM>O equivalent basis vs NaOH+Na<NUM>O) to be expressed as % of starting solids recovered in liquid phase.

Recovery was interpreted as the cumulative solids recovered from <NUM>st and <NUM>nd centrifuging with the understanding that in a commercial setting if the diluted liquid from the solid washing did not meet commercial specification it could be:.

Recovery of Na, which was added as NaOH but recovered as Na<NUM>O equivalent, was made and is denoted as such where appropriate.

From a commercial perspective SiO<NUM>:Na<NUM>O, purity and final concentration of SiO<NUM> in solution are most important factors. It is expected that product formation (e.g. C-S-H) and possible evaporative losses removed water from solution during the experiments; distorting the concentration of ions in solution as a measure of digestion/dissolution. Consequently, results are presented in terms of accumulation of SiO<NUM> in the liquid phase with units of g L-<NUM>.

Accumulation of SiO<NUM> in the liquid phase increased with higher temperatures (<FIG>). Concentrations increased in a linear fashion for all treatments until <NUM> hours (<NUM> SiO<NUM> L-<NUM>) and <NUM> hours (<NUM> SiO<NUM> L-<NUM>) for <NUM> and <NUM> respectively, with <NUM> maintaining a linear increase in concentration for the duration of the experiment (<NUM> hours). Fitted line models for this initial linear period displayed in Table <NUM> estimate accumulation of SiO<NUM> in the liquid phase was <NUM> and <NUM> times higher for <NUM> than <NUM> and <NUM> respectively. Four data points are missing between <NUM> and <NUM> hours for <NUM> as samples were highly viscous and unable to be analysed. Highest concentrations were reached for each treatment after <NUM> (<NUM> SiO<NUM> L-<NUM>), <NUM> (<NUM> SiO<NUM> L-<NUM>) and <NUM> (<NUM> SiO<NUM> L-<NUM>) hours for <NUM>, <NUM> and <NUM> treatments respectively.

Similar to the results in the first set of experiments, in the second set of experiments the increase from <NUM> to <NUM> led to a 5x increase in the rate of SiO<NUM> accumulation in the liquid phase, with a temperature equal to or greater the <NUM> and less than boiling (nominally <NUM>) optimal for the reaction kinetics without requiring increased energy requirements and capital expenditure associated with pressurised systems. A lower maximum silica contents in the second set of extractions is likely to be related to either or both: <NUM>) a decrease in concentration of glass used in the reactor or <NUM>) Decreased evaporative losses from improved experimental controls. Nonetheless, recoverability was determined dissolved contents of the liquid retrieved from the mixture. These results are shown in Table <NUM> and represented graphically in <FIG>.

Comparison of paired treatments in <FIG> show that higher starting glass concentrations for treatments in the range <NUM>-<NUM> glass L-<NUM> H<NUM>O led to a <NUM>% or greater increase in the accumulation rate of SiO<NUM> in solution (Table <NUM>). All treatments demonstrated a linear increase in concentration during the experiment, except for <NUM> L-<NUM>/<NUM> NaOH/<NUM> where linear increase was recorded until <NUM> hours (<NUM> SiO<NUM> L-<NUM>). After this point several samples were taken which were unable to be processed as discussed above and water was added to allow supernatant extraction. Treatments with higher loadings (<NUM> glass L-<NUM> H<NUM>O) were initiated but results were invalidated due to equipment failure. The data generated from them did not suggest a large difference in dissolution rate for <NUM> glass L-<NUM> H<NUM>O treatment when compared to <NUM> glass L-<NUM> H<NUM>O but further tests are to be conducted.

As shown in <FIG>, SiO2 accumulation in the liquid phase during the linear accumulation phase was roughly proportional to the concentration of glass, with an increase of <NUM>-<NUM>. 1x for each of the concentration increases from <NUM> → <NUM> → <NUM> → <NUM> glass L-<NUM> H<NUM>O. Unlike the previous set of experiments, <NUM>/<NUM>/<NUM> treatment was successfully completed and showed both the fastest accumulation of SiO<NUM> in the liquid phase. It is anticipated that the accumulation rate of SiO<NUM> in liquid phase will continue to increase until physical challenges arise in keeping digester contents suspended and well mixed. As discussed later, the relatively poor recoverability of solids seen in the <NUM>/<NUM>/<NUM> treatment indicates that this is likely the upper limit of commercially relevant reaction conditions.

Accumulation of SiO<NUM> in the liquid phase was > <NUM>% faster when NaOH was used as the alkali source as opposed to KOH (<FIG>, Tables <NUM> and <NUM>). <NUM> concentrations had slightly higher (<NUM>% and <NUM>% for NaOH and KOH respectively) accumulation rates of SiO<NUM> in solution than <NUM> for the duration of <NUM> experiment (<FIG>).

<FIG> shows that unlike glass concentration, accumulation rate of SiO<NUM> in the liquid phase does not increase proportionally with increasing concentrations of alkali metal hydroxide. Rather, there is an almost identical accumulation rate for the first <NUM> hours for all treatments before the <NUM>/<NUM>/<NUM> diverges, indicating that NaOH concentration is limiting. In contrast, <NUM>/<NUM>/<NUM> and <NUM>/<NUM>/<NUM> treatments have limited divergence with the accumulation rate of y=<NUM>. 422x and y=<NUM>. 279x respectively and what they appear to be limited by a similar maximum concentration asymptote.

As expected, treatments with lower NaOH or KOH starting additions showed a faster increase in SiO<NUM>:Na<NUM>O+K<NUM>O (<FIG>. The relative difference between paired treatments was greater for mole ratio (table <NUM>) than SiO<NUM> accumulation (Table <NUM>) for the same runs.

As previously noted above, SiO<NUM> g L-<NUM> shows a linear accumulation over the first <NUM> hours (y=<NUM>. 67x, R<NUM>=<NUM>). After this it plateaus, decreased between <NUM> hours and <NUM> hours and then then increases after the addition of water for the final sampling points at <NUM> and <NUM> hours. <FIG> shows that while the SiO<NUM> concentration plateaus between <NUM> and <NUM> hours the SiO<NUM>:Na<NUM>O ratio continues to increase in this period, albeit more slowly, to reaching a local maximum of <NUM>:<NUM> at <NUM> hours. <NUM>-<NUM> sees a slight decline to <NUM>:<NUM> before a steep increase to <NUM>:<NUM> following water addition at <NUM> hours and a final value of <NUM>:<NUM> at <NUM> hours. Solids content shows a similar pattern to SiO<NUM> accumulation in solution; increasing rapidly to <NUM>% at <NUM> hours before reaching a maximum of <NUM>% at <NUM> hours and then declining, in line with lower total solubility of higher SiO<NUM>:Na<NUM>O ratio solutions, to <NUM>% at <NUM> hours and increasing slightly to <NUM>% at <NUM> hours.

Solids recovery increased varied from <NUM>-<NUM>% between treatments using the method described above. No measure of error is possible for this data as each recovery represents harvesting of an individual treatment (time x reaction conditions). Optimisation of the washing conditions as known in the art (both number and procedure) is expected to increase recoverability of sodium silicate in the liquid phase.

The average purity of final liquid fractions in the first set of digestions, measured as SiO<NUM>+Na<NUM>O+K<NUM>O's proportion of total dissolved metal oxides, was <NUM> with a <NUM>% confidence interval of ±<NUM>%. For the digestions completed in triplicate, average purity of final solution was <NUM> (±<NUM>). Iron, commonly the most problematic impurity in sodium silicates, had a maximum concentration of <NUM> ppm in the first set of digestions and an average value of <NUM> ppm for the final solution in second set of digestions. This compares to the ~<NUM> ppm normally present in sands (Lagaly, <NUM>).

Aluminium was the most common element found in solution with an average of <NUM>% of dissolved metal ions on a metal oxide equivalent basis/.

A summary of the amounts and ratios of SiO<NUM> and Na<NUM>O are set out in Table 11A:.

<NUM> of moist CSH was added to a <NUM> Falcon tube and suspended in <NUM> Milli-Q water. To this <NUM> HCl was added dropwise with a <NUM> pipette in the following intervals: <NUM>, <NUM>. Suspension was then centrifuged and pH measured before an additional <NUM> Milli-Q water was added to facilitate stirring. Acid was then added in the amounts: <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>. The sample was stirred, and centrifuged after each addition and the pH recorded. Photographs of the sample at various stages of the process were taken and amount of HCl added plotted against pH to determine when the buffering capacity of the CSH was exhausted.

pH was determined after the final acid addition and the supernatant was transferred to a separate <NUM> Falcon tube. To this <NUM> NaOH was added in the following increments: <NUM>, <NUM>, <NUM>. After the final addition there was no further precipitate with additional NaOH addition and the sample was centrifuged and the centrate discarded with the solid fraction sample kept for further analysis.

As shown in <FIG>, pH declined linearly on the log pH scale, with the first visible signs of stable dissolution occurring at pH <NUM>. The decline followed the same linear trend with the addition of acid from pH <NUM> to <NUM> with each acid addition improving the clarity of the gel, a result which was more apparent to the eye than in photographs. Optimal clarity was reached at <NUM> pH with no apparent cloudiness of the gel. The rapid decrease in pH relative to the amount of acid added at this point was interpreted as the exhaustion of the buffering capacity of the CSH suspension.

In a series of further experimental runs, CSH was prepared by adding <NUM> L-<NUM> of glass to <NUM> of <NUM> NaOH and digesting in triplicate for <NUM> hours. After <NUM> hours the contents were placed into <NUM> Falcon tubes and centrifuged at <NUM> rpm for <NUM> minutes; fractionating the mixture into liquid (sodium silicate), digested (calcium silicate hydrate) and undigested/partially digested fraction (undissolved/partially dissolved glass).

To generate sufficient CSH for the leaching experiment the undissolved/partially dissolved solid fraction was re-suspended in <NUM> of <NUM> NaOH for another <NUM> hours before separating the CSH. This step was then repeated once more with the remaining partially/undissolved fraction as above.

To ensure purity of CSH solid fraction CSH was resuspended in water equal to 2x the weight of the gel fraction before once again separating liquid, C-S-H, and partially/undissolved fractions as above. Once completed all CSH was mixed thoroughly to ensure consistency of composition and then stored in a sealed HDPE container.

In further tests that were carried out, <NUM> of CSH was suspended in <NUM> of water and pH was measured via in-vessel pH probe after additions of <NUM> HCl in <NUM> increments until pH was <<NUM> (<FIG>).

pH was recorded after one further <NUM> addition of <NUM> HCl and then, due to large drops in pH followed by slow rebounds to ~pH <NUM> (hypothesised to be caused by exhaustion of 'free' solution alkalinity vs slower release of 'bound' solid alkalinity) HCl was added drip-wise in <NUM> increments to maintain solution between <NUM> and <NUM>, until pH remained stable and <<NUM> after <NUM> minutes.

Liquid and solid fractions were separated by centrifuging for <NUM> minutes at <NUM> rpm removing centrate. Solid fraction was then 'washed' twice via suspension in 2x the wet weight of solid followed by centrification for <NUM> minutes at <NUM> rpm. Liquid samples were collected at each stage (Centrate, Wash <NUM> & Wash <NUM>) and kept for analysis as shown in table <NUM>. All samples were then analysed using ICP-OES with solid samples microwave digested using a <NUM>:<NUM>:<NUM> ratio of HNO3, HCl and HF acids. Composition of solids is calculated as equivalent weight% of element oxides measured (Na, Ca, Si, K, S, Mg, Al, Fe, P, Zn, P & B).

Results show that SiO<NUM> gel of ~<NUM>% purity can be generated via leaching of CSH using the method detailed above. Given the concentration of non-silica elements (namely Na & Ca) in wash <NUM>, and the high purity levels (><NUM>% SiO2) previously achieved with washing, it is expected that higher levels of purity will be easily achievable with further washing of the solid.

Likewise, due to the extended digestion method used to generate CSH, Na contents was higher than that previously measured (circa <NUM>% CaO and <NUM>% Na<NUM>O vs <NUM>% CaO and <NUM>% Na<NUM>O).

The efficacy of the process across a range of feedstocks show that the process is suitable as a means to extract silica gel from the range of 'calcium silicate hydrates' able to be generated from soda-lime glass via alkaline digestion.

The results demonstrate, for the first time, that it is possible to separate waste glass into a commercially relevant sodium silicate solution and calcium silicate hydrate gel using a low-energy alkaline digestion process. Further, SiO<NUM>:Na<NUM>O of the solution can be manipulated by both retention time and starting Glass:NaOH ratio to produce a range of sodium silicate solutions from <NUM>-<NUM>:<NUM> with solids concentrations of <NUM>-<NUM>% (above this the samples were too viscous to be processed). The relatively high value of the sodium silicate, the minimal cost of inputs, positive environmental impact and lower energy requirements compared to traditional production routes make this an attractive target for further commercialisation testing.

An approximate mass-balance leaching test, conducted by adding at total of <NUM> of water to <NUM> samples of the slurry produced at the end of run no. <NUM>, indicated that ~<NUM>% of starting solids (glass+NaOH) were recoverable as dissolved Na<NUM>O or SiO<NUM>. This is relatively low compared to the estimate of Maraghechi et al. (<NUM>) who, after investigating the dissolution rates of different glass types in the context of improving pozzolanic and ASR reactions in the addition of glass to concrete, estimated that ><NUM>% of dissolved Si was present in the liquid phase, with the remainder being partitioned into solid corrosion products such as C-S-H.

A lower SiO<NUM>:Na<NUM>O ratio in the leached samples (<NUM>-<NUM>:<NUM>) of the mass-balance compared to the centrifuged samples (<NUM>:<NUM>) at the same time point also indicated that Na2O is preferentially leached when water is added in excess. This is relevant in a commercial context as centrates are likely to be the optimal process to harvest high SiO<NUM>:Na<NUM>O ratio solutions, whereas leaching appears better suited for the washing and reticulation of dissolved Na and Si from solid fraction.

The dissolution of amorphous silicon and formation of C-S-H have both been demonstrated to be exothermic reactions, generating <NUM> MJ per t and, depending on hydrated mineral type, <NUM>-<NUM> MJ per t starting solids respectively (Gunnarsson and Arnórsson, <NUM>, Stutzman et al. Enthalpy of the process, due to its novelty, is yet to be measured directly. However, a conservative estimate for the release of energy in the digestion process would be <NUM> MJ per t glass or <NUM> kW h per t glass (the same amount of energy required to raise the temperature of <NUM> of water +<NUM>). Given an industrial scale digester is likely to be coupled with a heat exchanger, operating at an efficiency of ><NUM>% (Whitaker, <NUM>), the process should largely self-sufficient in meeting heating requirements once started.

The use of soda-lime glass as a feedstock for sodium silicate the production would provide a major disruption to sodium silicate production and waste glass disposal supply chains. <FIG> shows a schematic representation of current conventional processes for manufacturing glass and producing waterglass. <FIG> provides a schematic representation of the changes that may arise using embodiments of the present invention.

<FIG> and <FIG> show possible process flow diagrams for embodiments of the present invention. <FIG> shows a process flowsheet for a continuous process in accordance with an embodiment of the present invention. In <FIG>, a waste glass hopper <NUM> supplies waste glass to digestion vessel <NUM>. Sodium hydroxide <NUM> is added to digestion vessel <NUM> to cause digestion of the glass, formation of sodium silicate and formation of CSH. The mixture of products from digestion vessel <NUM> is removed at <NUM> and sent to solid/liquid separation <NUM>. The liquid sodium silicate <NUM> is sent for further processing, use or storage. The solid fraction <NUM>, which includes undigested glass and CSH, is sent to solid/solid separator <NUM> where the undigested glass is separated from the CSH on the basis of particle size and density differences. The undigested glass <NUM> is returned to the digestion vessel <NUM>.

The CSH is sent to vessel <NUM> where it is contacted with acid <NUM> to form SiO<NUM>. The solid SiO<NUM> or SiO<NUM> gel is removed at <NUM>. Part <NUM> of the SiO<NUM> is sent for further processing, use or storage. Part of the SiO<NUM> <NUM> is returned to digestion vessel <NUM>. Liquid stream <NUM>, which contains dissolved metal salts, is removed from vessel <NUM>.

Particle size distribution is expected to have a large impact on reaction kinetics given the inverse relationship between reactive surface area and particle size. However, as:.

It was determined that this was not a pressing parameter for investigation in this or subsequent digestions.

The production of commercial sodium silicate solution from a low-energy alkaline digestion in accordance with embodiments of the present invention is possible and the technique appears to have a number of economic and environmental benefits compared to traditional manufacturing routes. In preferred embodiments of the present invention, a waste glass product may be used as a feed material, thus lessening the amount of this waste material that would otherwise go into landfill. Further, the amount of energy required to produce waterglass in accordance with embodiments of the present invention is reduced when compared to conventional processes. The economics of the embodiments of the process of the resent invention may be further enhanced by recovering calcium-silicate-hydrate for sale or use.

In some embodiments of the present invention, sodium silicate solutions of varying SiO<NUM>:Na<NUM>O+K<NUM>O can be obtained by:.

Possible uses for the products of embodiments of the present invention include:.

While extensive research has been conducted into mixing soda-lime glass and alkali metal hydroxides to create 'alkali-activated geopolymers', the extraction of high purity, commercially equivalent sodium silicates solutions from soda lime glass at normal atmospheric pressure has not yet been explored. Previous workers have focused on dissolution of glass at standard atmospheric pressure with the aim of creating alkaline activators of materials such as slag or fly ash for the substitution of Ordinary Portland Cement (OPC). However, to the knowledge of the inventors, embodiments of the present invention represent the first disclosure of a low-energy process by which a high-purity and concentration sodium silicate, suitable to replace commercially available sodium silicates is produced from waste glass.

One of the major advantages of the process of the present invention lies in its simplicity. However, this masks a number of non-obvious and inventive steps taken during the conception of the process. These include:.

Glass is not, at first glance, an ideal silicon/sodium feedstock for waterglass production. Previous alkali-activated geopolymer studies have not emphasised purity or yield of silicon over a given time frame as focus has been given to the durability of the final cementitious material. This is largely because CaO is added to soda-lime glass specifically to promote stability and insolubility and at the same time soda-lime glass contains a high degree of contaminants (<NUM>-<NUM>% of materials not Na2O or SiO2) when compared to traditional SiO2 feedstocks (Quartz= ~<NUM>% SiO2) for sodium silicate. However, in preferred embodiments of the present invention, under the right reaction conditions the preferential accumulation of contaminants in the solid phase results in a separable liquid phase which is a commercially acceptable product.

Previously reported optimal glass dissolution conditions for geoploymer alkaline activators are <NUM>/<NUM> hours, a point after which limited dissolution appeared to take place. It is also evident from the literature that additional glass results in a lower relative yield of SiO<NUM> over the same time period. Thus, it is not an obvious step to increase glass concentrations by multiple times relative to previous studies or to extend dissolution times to allow for greater digestion.

To the best knowledge of the inventors, all commercial waterglass manufacturing techniques prior to the present process have required the use of a combination of high temperature and pressure (traditional hydrothermal route) or extremely high temperature (furnace) in order to form a commercial sodium silicate solution within an economically viable timescale. The ability of a hydrothermal process, even with high temperature and pressure, to directly form sodium silicates with SiO<NUM>:Na<NUM>O ratios ><NUM>:<NUM> is unusual and ><NUM>:<NUM> unprecedented. The present inventors have found there is a small window within which reaction kinetics and final desired product can be optimised with respect to one another and ideal parameters, as used in embodiments of the present invention, were only deduced after multiple unsuccessful attempts which started based on the existing literature. Given the degree to which the process reshapes understanding of sodium silicate manufacturing routes there is little chance it would have been obvious to a skilled person.

Claim 1:
A method for the alkaline digestion of soda-lime glass, the method comprising:
(a) forming a mixture of soda lime glass and a hydroxide solution, the mixture having at least <NUM> grams of glass per litre of H<NUM>O, the hydroxide solution having a concentration of <NUM> or greater to thereby form, by digesting the glass in the mixture:
- an aqueous sodium silicate fraction having a silicate concentration of <NUM>/L or greater calculated as SiO<NUM> equivalent and a ratio of SiO<NUM>:M<NUM>O of at least <NUM>, wherein M<NUM>O is an alkaline metal oxide; and
- a solid material comprising a calcium-silicate-hydrate;
(b) separating the aqueous sodium silicate fraction from the solid material comprising the calcium-silicate-hydrate;
(c) treating the solid material comprising the calcium-silicate-hydrate with an acid to dissolve soluble metals from the calcium-silicate-hydrate, thereby producing a liquid phase and a solid phase, wherein the solid phase comprises SiO<NUM>;
(d) separating the solid phase comprising SiO<NUM> in step (c) from the liquid phase; and
(e) returning at least some of the separated solid phase comprising SiO<NUM> to step (a).