Method for Management of Contaminants in Alkaline Process Liquors

Field of the Invention

The present invention relates to a method for the management of contaminants in alkaline process liquors.

Background Art

The application of the present invention will be discussed in the context of the removal, disposal and management of contaminants of Bayer process solutions, although the scope of the invention encompasses the management of contaminants of any alkaline solution, including the black Kraft liquors associated with the pulp and paper industry.

The Bayer process is widely used for the production of alumina from alumina- containing ores, such as bauxite. The process involves contacting alumina- containing ores with recycled caustic aluminate solutions, at elevated temperatures, in a process commonly referred to as digestion.

The sodium aluminate solution so produced also contains insoluble residues from the bauxite ore, and the solids are separated from the solution in a thickener or clarifier. The solids, known as 'red mud', are taken as underflow from the thickeners and then typically washed to recover caustic values and render the mud suitable for disposal. The dense slurry is pumped to drying beds and distributed over a surface to allow the residue to dry atmospherically, with entrained caustic liquor being recovered via an underdrain system. The underdrain caustic liquor is recycled for further use in the Bayer circuit along with spent liquor from other locations in the circuit.

After cooling the thickener overflow, aluminium hydroxide is added as seed to induce the precipitation of further aluminium hydroxide therefrom. The precipitated aluminium hydroxide is separated from the caustic aluminate solution, with a portion of the aluminium hydroxide being recycled to be used as seed and

the remainder recovered as product. The remaining caustic aluminate solution is recycled for further digestion of alumina-containing ore.

In some regions of the world, a significant amount of organic material (also known as total organic carbon or TOC) accompanies the bauxite, a portion of which is responsible for the presence of a range of organic compounds in the resulting solutions. The presence of organic compounds in Bayer process solutions reduces productivity largely through two effects. Firstly, organic compounds combine with free soda reducing the soda available to dissolve alumina and form sodium aluminate in solution. The solubility limit of certain organics ultimately restricts the overall concentration of soda in the liquor circuit. Secondly, the presence of organic compounds reduces the hydrate precipitation rate, due to crystallisation poisoning. Benefits associated with removal of organic compounds from Bayer process solutions include a reduction in the amount of soda in the alumina product, reduced liquor viscosity and improved hydrate agglomeration. Subsidiary disadvantages associated with organic compounds of Bayer process solutions include reduced boiling point, foaming, liquor and hydrate absorbance and liquor density.

The presence of inorganic impurities dissolved during digestion has two main impacts on the efficiency of the Bayer circuit. Firstly, some impurities pass into the precipitated hydrate. Secondly, impurities recirculate within the circuit and can result in suppression of the alumina yield. An example of an inorganic impurity removal system is the desilication process. Bauxites are known to comprise 'reactive silica ¹ in various amounts. Reactive silica dissolves readily in caustic and subsequently reacts with caustic and alumina to form 'desilication product'. It is also known to add reactive silica to bauxite to encourage precipitation of desilication product. Whilst the precipitation of desilication product can remove sulphate and other impurities from the circuit, it consumes caustic and alumina and does not provide a usable end product. Moreover the precipitated desilication product incorporated in residue can make residue neutralisation more problematic.

Refineries employ several methods to reduce or control the levels of contaminants, including organic compounds in alkaline solutions in process liquor. These include specific removal processes, controlled entrainment of liquor to residue disposal and natural loss associated with organic adsorption onto product hydrate.

A specific organic, sodium oxalate (oxalate) forms a substantial component of the overall organic component in process liquor and is targeted for removal in refineries. It builds up in the liquor stream as a result of direct input from bauxite and from the natural degradation of other organics as the liquor is continually recycled through the Bayer circuit.

Current oxalate disposal operations involve conversion of sodium oxalate with slaked lime to produce calcium oxalate that is transferred to a residue area as a slurry. A significant amount of the calcium oxalate is naturally reconverted to soluble sodium oxalate and returns to the process with lake water return. The remaining calcium oxalate is incorporated in the mud residue. The amount of sodium oxalate returning to the refinery process circuit, as a result of reversion back to sodium oxalate, has a significant detrimental effect on the process. Alternate methods for oxalate removal include destruction in kilns, storage or biological destruction.

Practically, processes for removing contaminants from Bayer circuits are specific to each contaminant (e.g. sulfate by desilication product formation, and oxalate removal). The use of specific processes requires large amounts of capital and adds to the complexity of the circuit. Some contaminants (e.g. chloride, fluoride and gallium) have no effective control measures.

The Bayer process uses large amounts of costly caustic solution and there is considerable focus on recycling caustic solutions from one part of the circuit to another for further use. Caustic streams with contaminants are not removed from the circuit and disposed of due to environmental and cost considerations. Much

research is conducted into developing more efficient ways to remove contaminants from Bayer liquors in order that caustic solutions may be recycled.

As aluminium hydroxide is precipitated and bauxite dissolved, the concentrations of sodium hydroxide present in the process solution decrease, whilst concentrations of sodium carbonate increase, reducing the efficacy of the solution for digestion of further aluminium-containing ore. Accordingly, processes aimed at improving the ratio of hydroxide to carbonate (causticising) Bayer liquors have been developed in order that caustic solutions may be recycled.

Similarly, there is a heavy emphasis in the paper industry on the recycling of costly caustic solutions and much effort is expended in removing contaminants from process streams so that the caustic may be recycled.

Alumino-silicate cementatious binders, herein termed "geopolymers", are mineral polymers and have been shown to have higher strength than Ordinary Portland Cement, have a high acid resistance, have greater stability at higher temperatures than Ordinary Portland Cement and their production results in liberation of much less green house gas than Ordinary Portland Cement.

Geopolymerisation is a geosynthesis that involves alumino-silicates. The silica and alumina units react to form XRD amorphous rigid structures that are chemically and structurally comparable to those binding natural rock. The atomic ratio of Si and Al in the structure determines the properties of the geopolymer.

Geosynthesis reactions require large amounts of caustic which adds significantly to the cost of formation. Further, dissolution of sources of alumina into caustic solutions can be difficult, with non crystalline or amorphous forms widely favoured. (Shvarzman A; Kovler, K; Grader, G; and Shter, G; The effect of dehydroxylation/amorphization degree on pozzolanic activity of kaolinite in Cement and Concrete Research 33, 2003, 405-416)

Whilst sources of alumina and silica are available, (e.g. fly ash and mete-kaolin), to activate the polymerisation process, at least the surfaces of these particles need to partially dissolve in highly concentrated caustic solutions under tight specified conditions, followed by aluminosilicate formation. Yong, S; Feng, D; Lukey, G; and Van Deventer J., (The Effect of Partially Reacted Surfaces on the Short Range Ordering of Geopolymer Gels in World Congress Geopolymer 2005 Conference, France) stated that "A geopolymer is essentially a composite material consisting of (a) partially reacted particles (e.g. ash or metakaolin) (b) the newly formed aluminosilicate (geopolymer) gel: and (c) sand, aggregate, fibre etc."

The formation of geopolymers was patented by Davidovits in US 4,349,386 (1982), US 4,472,199 (1984) and US 4,509,985 (1985). These patents and associated articles since have reported a set of conditions that are generally required for the formation of a geopolymer. These conditions can be written as a series of idealised ratios that if adhered to will form a polymer although those skilled in the art will appreciate that the ratios are guides only.

The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Throughout the specification, unless the context requires otherwise, the word "solution" or variations such as "solutions", will be understood to encompass slurries, suspensions and other mixtures containing undissolved and/or dissolved solids.

Throughout the specification, unless the context requires otherwise, the words formate, acetate, oxalate, malonate and succinate, and will be understood to refer to all anions, organic acids and salts of formate, acetate, oxalate, malonate and succinate.

Disclosure of the Invention

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

The present invention is not to be limited in scope by the specific embodiments described herein, which are intended for the purpose of exemplification only. Functionally equivalent products, compositions and methods are clearly within the scope of the invention as described herein.

The entire disclosures of all publications (including patents, patent applications, journal articles, laboratory manuals, books, or other documents) cited herein are hereby incorporated by reference.

In accordance with the present invention, there is provided a method for the management of contaminants in an alkaline process circuit, the method including the steps of:

adding a source of silica to an alkaline process liquor from the alkaline process circuit;

adding a source of alumina to the alkaline process liquor; and

forming a geopolymer.

It will be appreciated that the present invention will have application in any industry utilising alkaline liquors in a process circuit wherein there is a focus on recycling of the liquor. The alkaline process stream may be provided in the form of Bayer process stream or a Kraft process stream.

In accordance with the present invention, there is provided a method for the management of contaminants in a Bayer process circuit, the method including the steps of:

adding a source of silica to a Bayer process liquor from the Bayer process circuit; and

forming a geopolymer.

In the context of the present invention, the term "management of contaminants" shall be taken to encompass the removal of contaminants from a process circuit. In addition, the term may also be taken to encompass the purification of liquors in the process circuit by contaminant removal. In the context of the specification, the term "purification" shall be taken to encompass decreasing the level of contaminants in the process circuit.

Phair J & Van Deventer J (Characterization of Fly Ash Based Geopolymeric Binders Activated with Sodium Aluminate in American Chemical Society Web published 07/17/2002) have demonstrated that geopolymers can be made from pure sodium aluminate solutions, but failed to recognise the counter intuitive value in using a concentrated Bayer liquor, nor the implications that a concentrated liquor would provide an impurity bleed for the alumina refinery.

Advantageously, the present invention provides a single process for removing contaminants from the alkaline process circuit. Prior art processes for removing contaminants from Bayer circuits, for example, are either specific to a certain contaminant or non-existent.

It will be appreciated that the removal of contaminant-containing liquors from the alkaline process circuit as a means of managing contaminants in the circuit is antithetical to the practice of those industries such as alumina and paper, where there is a heavy focus on recycling caustic solutions.

Advantageously, the formation of a geopolymer offsets the disadvantages associated with removing caustic process liquor from the process circuit.

It will be appreciated that the contaminants that are removed by the method of the invention are not limited to organic contaminants and may include inorganic contaminants. Further, the types of contaminants removed will be influenced by the source of the liquor within the circuit.

With respect to the Bayer Process, the present invention advantageously enables bauxites higher in contaminants (for example, soluble salts and organics) to be treated in the circuit. Utilising the method of the present invention and removing a caustic stream from the circuit and adding fresh caustic to the circuit will decrease the concentration of contaminants, and using said stream in the formation of a geopolymer can offset the economics associated with the treatment of low grade bauxite. It will be appreciated that when processing bauxites higher in contaminants, the caustic stream need not be predigestion liquor.

Advantageously, the present invention provides a stabilised solution of caustic alumina.

The present invention enables the properties of the geopolymer to be tailored by appropriate choice of liquor within the process circuit. The present invention also enables the properties of geopolymer reagents to be tailored by appropriate choice of liquor.

As used herein the term "source of silica" shall be taken to include, without limitation, any form of silica that provides a silicate anion in Bayer process solutions.

The source of silica may be selected from the group comprising sodium silicate, kaolin, meta kaolin, silica flour, fly ash, glass, blast furnace slag, bauxite ,laterite, feldspar, silicaceous clay and desilication product.

Advantageously the present invention may be able to utilise low alumina bauxite or low reactive alumina fly ash which prior art methods of geopolymer formation cannot utilise without an additional alumina source. The choice of starting alumina liquor may provide sufficient soluble alumina to allow binding to take place.

It will be appreciated that the ratio of silicon to aluminium will depend on the ultimate proposed use of the geopolymer and will be influenced by the source of the Bayer process liquor.

Preferably, the total silica/alumina ratio is about 1 or more.

In one form of the invention, the method includes the further step of:

adding a source of alumina to the Bayer process liquor.

The source of alumina may be provided in the form of alumina, kaolin, meta kaolin, fly ash, bauxite or laterite.

It will be appreciated that liquors from various parts of the Bayer circuit may be utilised in the present invention and that different liquors with different types and concentrations of contaminants as well as different concentrations of caustic and alumina will affect the geopolymerisation reaction as well as the final geopolymer properties. For example, spent liquors, green liquors, lake waters, liquor burner feeds, evaporator discharge, membrane retentate and desilication liquors may be utilised. Advantageously, desilication liquor may comprise dissolved silica which may be utilised in the geopolymerisation reaction.

Preferably, the method comprises the further step of:

retaining at least a portion of the contaminants in the geopolymer.

The geopolymer may physically or chemically retain the contaminants. Without being limited by theory, it is believed that the polymerisation reaction or reactions may chemically bind some contaminants within the geopolymer structure. It will be appreciated that the propensity of the contaminant to chemically interact in the polymerisation reaction will depend on the properties of the contaminant.

Without being limited by theory, it is believed that the presence of dissolved species in the process liquor may advantageously affect the properties of the geopolymer, depending upon liquor selection. For example, organics, organic acid salts (carboxylate anions) and carbonates may modify the set time of geopolymers potentially by interaction with the polymeric species and hence the polymerisation process. They may also modify the reagent slurry rheology as well as the final product properties, such as level of efflorescence. Further, it is believed the organic contaminants may improve the coordination of the geopolymer, potentially adding strength to the polymer as well as affecting conductivity and density of the geopolymer.

Without being limited by theory, it is believed that the organic contaminants in the liquor may slow the geopolymerisation reaction or reactions and add time to geopolymer setting which may be advantageous when transporting geopolymer mixes. It is believed that the contaminants may tie up reactants in the mix thereby hindering the polymeric-like formation. In the event that it becomes necessary to delay setting of a geopolymer mix, for example, during transport, a concentrated solution or powder of contaminants may be added to the geopolymer mix. It is known that the addition of excess water to a geopolymer mix will stop any geopolymerisation reaction.

Without being limited by theory, it is believed that the presence of contaminants other ions (species) may impact on the efflorescence of the geopolymer and possibly also provide increased resistance to acid and/or alkali attack. For example, some cations such as iron and gallium may substitute for aluminium or

silicon in the geopolymer framework and some organics such as formate or oxalate or other small anions or cations such as quaternary ammonium ions may template the framework as is known in zeolite formation. Some contaminants such as small organic anions may chelate inorganic species thereby immobilising them into the geopolymer structure and inhibiting leaching. Specifically, the presence of organic acid salts (carboxylate anions) may immobilise soda during the geopolymer setting process and prevent its subsequent efflorescence, a particularly difficult and visible barrier to commercial acceptance of geopolymers. The effect will be akin to the use of carbonate, also present in the liquor and believed likely to impart the same effect on possible reduction in efflorescence.

Preferably, the addition of a source of silica to the Bayer process liquor forms a paste. Without being limited by theory, it is believed that the contaminants may improve paste workability and rheology not necessarily by making the paste thicker but by increasing its ability to hold shape. Further, entrapment and inclusion of bubbles may assist foam rock production.

It is believed that the high concentrations of inorganic salts, carbonate and carboxylate anions present in Bayer process liquors may affect the activity of the water and the caustic. Solvation of contaminants can result in lower levels of free water available for the geopolymer reaction, possibly allowing for wider ranges of geopolymer formation conditions.

The concentration of impurities within the Bayer process liquor is such that the activity of water is reduced. Without being limited by theory, it is believed that this may widen the specifications of allowable water content during geopolymer formation.

Preferably, the method comprises the further step of:

adding a caustic solution to the alkaline process circuit.

The caustic solution is preferably provided in the form of a concentrated aqueous solution of an alkali metal or alkaline earth hydroxide. In a highly preferred form of the invention, the caustic solution is provided in the form of a concentrated aqueous solution of sodium hydroxide. Where the caustic solution is provided in the form of a concentrated aqueous solution of sodium hydroxide, the sodium hydroxide solution may be a 50 wt% solution.

Where alkaline process circuit is a Bayer process circuit and the method comprises the step of:

adding caustic solution to the Bayer process circuit,

the caustic solution is preferably added to the Bayer process circuit prior to the step of digesting bauxite.

In one form of the invention, the method comprises the further step of:

adding caustic to the alkaline process liquor prior to the step of forming the geopolymer.

The caustic may be provided as an aqueous solution or a solid. Where the caustic is provided as an aqueous solution, the caustic is preferably provided in the form of a concentrated aqueous solution of an alkali metal or alkaline earth hydroxide. In a highly preferred form of the invention, the caustic is provided in the form of a concentrated aqueous solution of sodium hydroxide. Where the caustic is provided in the form of a concentrated aqueous solution of sodium hydroxide, the sodium hydroxide solution may be a 50 wt% solution.

In one form of the invention, the method further comprises the step of:

dewatering the alkaline process liquor.

The step of dewatering the alkaline process liquor may comprise evaporating water from the alkaline process liquor. It will be appreciated that the requirement of dewatering the process liquor will be influenced by the choice of liquor.

It will be appreciated that the temperature of the process liquor may have an affect on the geopolymerisation reaction and that the temperature of the process liquor will be influenced by the choice of process liquor.

Preferably, the step of forming a geopolymer is performed between about 20 ⁰ C and 110 ⁰ C. More preferably, the step of forming a geopolymer is performed between about 30 ⁰ C and 80 ⁰ C. More preferably still, the step of forming a geopolymer is performed between 60 ⁰ C and 80 ⁰ C.

In one form of the invention, the method comprises the further step of:

causticising the process liquor to convert carbonate to caustic.

It is known that geopolymerisation reactions are impacted by the level of caustic and it may be beneficial to convert the carbonate to free caustic through the further process of causticisation.

It will be appreciated that it may be necessary to add further caustic to the alkaline process circuit to account for the liquor removed from the circuit. It is believed that the advantages associated with geopolymer formation, including the ability to remove contaminants from the alkaline process circuit outweigh the need to add further caustic to the circuit to account for the caustic removed.

In accordance with the present invention, there is provided a method for the preparation of a geopolymer, the method including the steps of:

adding a source of silica to a Bayer process liquor from a Bayer process circuit; and

forming a geopolymer.

In one form of the invention, the method includes the further step of:

adding a source of alumina to the Bayer process liquor.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to one embodiment thereof, and the accompanying drawing, in which:-

Figure 1 is a schematic flow sheet showing how a method in accordance with the present invention may be utilised in a Bayer Process circuit.

Best Mode(s) for Carrying Out the Invention

Those skilled in the art will appreciate that the invention described herein is amenable to variations and modifications other than those specifically described.

It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

The invention focuses on the control of contaminants in a Bayer process by the removal of Bayer process liquors and the use of said liquors in geopolymer formation. The removal of liquors from a Bayer process circuit is contrary to all common practices. It will be appreciated that it is known to remove dilute process liquors from the circuit in red mud. The present invention teaches the removal of concentrated caustic liquors rather than those dilute liquors entrained in red mud.

Figure 1 shows a schematic flow sheet of the Bayer process circuit 10 for a refinery using a single digestion circuit comprising the steps of:

digestion 12 of bauxite 14 in a caustic solution;

liquid-solid separation 16 of the mixture to residue 18 and liquor 20;

alumina precipitation 22 from the liquor 20;

separation of alumina 22 and liquor 24; and

recycling liquor 26 to digestion 12.

As a non-limiting example, spent liquor 26 of approximate TC 230 gL ^"1 (concentration measured as gL ^"1 sodium carbonate) is concentrated through evaporation to an approximate TC of 550 gL ^"1 . This liquor is typically very dark due to the high concentration of organic species and syrupy due to the high concentrations of organics, salts, caustic, carbonate and aluminate. This evaporated liquor is typical of a Liquor Burner feed and can be used directly for the activation of an appropriate alumino silicate such as fly ash. Alternatively this liquor can be treated with calcium oxide to reduce the carbonate content and increase the free caustic, preferably prior to evaporation to allow easier separation of the resulting causticiser product solids.

Spent liquor 26 that has been evaporated up to high TA (550 gL ^"1 ) appears quite stable (over a period of days) without alumina precipitation at room temperature. It is believed the high concentration of free caustic, carbonate and organic species may help stabilise the alumina to remain in solution over this period.

It will be appreciated that other liquors may be used in geopolymer formation. Spent liquor 26 may be mixed with high concentrations of bauxite to dissolve silica and precipitate desilication product. This process could be interrupted to obtain liquor containing several gL ^"1 soluble silica. This process could be enhanced to elevate the silica levels for different application potentials.

It will be appreciated that the choice of liquor will be influenced by many factors including free caustic, carbonate levels, aluminate levels, silicate levels and concentrations of impurities that the refinery wishes to remove such as sulphate,

organics including oxalate, fluoride, anions and cations. Further, the choice of liquor may affect the final physical and chemical properties of the geopolymer. Higher concentrations of organics may impact upon efflorescence, as could carbonate levels. It is believed that salts and organics may change the setting rate of the geopolymer.

It will be appreciated that the removal of liquor from the alkaline process circuit results in all of the contaminants in the removed liquor being removed from the circuit at the same time. When fresh caustic is added to the alkaline process to replace the removed caustic liquor, the level of contaminants in the process circuit are decreased. By way of comparison, in the Liquor Burner example described above for the Bayer process, a typical flow of less than 1 % of total flow in the process circuit, is removed from the circuit and concentrated to a point to allow the liquor to be burnt, removing TOC and other volatile materials. However, the return of the resulting solids to the Bayer process results in the return of all the non-volatile impurities. In contrast, the present invention provides a method whereby the liquor that is removed from the circuit and used for the production of an alumino-silicate binder (geopolymer) is not returned to the circuit. By replacing the removed liquor with clean caustic, all the associated contaminants are effectively removed from the circuit.

It will be appreciated that the impurities removed by this process have many impacts upon the process, including improved product purity as well as process yield considerations.

The following Examples serve to more fully describe the manner of using the above-described invention, as well as to set forth the best modes contemplated for carrying out various aspects of the invention. It is understood that these Examples in no way serve to limit the true scope of this invention, but rather are presented for illustrative purposes.

Materials:

Spent liquor 26 was concentrated through evaporation to a TC of about 530 gl_ ^~1 TC (reported as Na ₂ CO ₃ ), TA of about 570 gl_ ^'1 (reported as Na ₂ CO ₃ ),) and sodium aluminate of about 215 gl_ ^"1 (reported as AI ₂ O ₃ ).

The spent liquor comprised organic contaminants such as but not limited to, formate, acetate, oxalate, malonate, succinate and higher molecular weight carboxylate anions and anionic inorganic contaminants such as sulfate, chloride, fluoride and cationic inorganic contaminants such as gallium.

Fly ash (Grade: ASTM C 618 (Class F)) was sourced from a power station in southwest Western Australia

Silica was provided in the form of Silica Flour 200 mesh from Unimin Australia Ltd.

Red sand was coarse bauxite residue sand obtained from the sand rakes at the Applicant's Kwinana Refinery in Western Australia.

Sodium silicate solution was provided in the form of a commercial sodium silicate solution (Waterglass) of density 1.4 g/mL.

Example 1

A portion of liquor 26 was prepared through evaporation having a TC = 531 gl_ ^"1 (reported as Na ₂ CO ₃ ), TA of 570 gl_ ^'1 (reported as Na ₂ CO ₃ ),) and sodium aluminate of 212 gl_ ^"1 (reported as AI ₂ O ₃ ).

Concentrated spent liquor (6 ml_) and fly ash (20.0 g) were stirred by hand with a nickel spatula for 5 min in a 50 ml_ plastic vial until a consistent homogenous paste was formed and the vial placed into an oven at 80 ⁰ C. After 7 days, the vial was removed and the mixture cooled to room temperature. The vial contained a hard solid that was difficult to scratch with a nickel spatula and remained intact when immersed in water for 8 days.

Example 2

Fly ash (18.0 g) and 10.0 % Unimin silica flour (2.0 g) were shaken together in a closed 50 ml_ plastic vial. Concentrated spent liquor (8 ml_) was added and the mixture stirred by hand with a nickel spatula for 5 min until homogenous. The vial was placed into an oven at 80 ⁰ C. After 7 days, the vial was removed and the mixture cooled to room temperature. The vial contained a hard solid that was difficult to scratch with a nickel spatula and remained intact when immersed in water for 8 days.

Example 3

Fly ash (18.0 g) and 10.0 % red sand (2.0 g) were shaken together in closed 50 ml_ plastic vial. Concentrated spent liquor (8 ml_) was added and the mixture stirred by hand with a nickel spatula for 5 min until homogenous. The vial was placed into an oven at 80 ⁰ C. After 7 days, the vial was removed and the mixture cooled to room temperature. The vial contained a hard solid that was difficult to scratch with a nickel spatula and remained intact when immersed in water for 8 days.

Example 4

Concentrated spent liquor (1 ml_) and sodium silicate solution (Water Glass P = 1.4 gml_ ^"1 1, 2.5 ml_) were stirred by hand with a nickel spatula for 5 min in a 50 ml_ plastic vial. A precipitate formed and the vial was placed in an oven at 50 ⁰ C for 72 hr.

Experiments have clearly shown that Bayer liquors may be used in the formation of alumino silicate binders (geopolymers). Hence a potential market exists for Bayer liquor bleed from the process as a method of contaminant removal and management.