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Controlled Release Zeolite Fertilisers: A Value Added Product Produced from Fly Ash Alex D. Elliot and Dong-ke Zhang Cooperative Research Centre for Coal in Sustainable Development, Centre for Fuels and Energy, Curtin University of Technology, GPO Box U1987, Perth, WA 6845 AUSTRALIA KEYWORDS: Zeolite, Agriculture, Fertiliser, Coal Fly Ash ABSTRACT Of 13 million tones of coal ash produced in Australia in 2003, only 34% is used in some way, with only 15% utilised in applications of value [1] , while the remainder is accumulated in landfills and ash dams. This low level of ash utilisation is inevitable due to the combination of inherently high transport costs, and relatively low value products. This situation argues for more value-added utilisation of coal ash to overcome the transport cost barrier. Zeolite synthesised from fly ash for agricultural application as a controlled release fertiliser, is a technology which offers considerable advantages in terms of economic, technical and environmental performance. The Australian fertiliser market consumed 192 Kt of potassium (K) in 1999 [2] . Assuming a cation exchange capacity of 3.5, this market is equivalent to 1.5 Mt of zeolite per annum, requiring roughly an equivalent amount of fly ash to produce. With Muriate of Potash (KCl, 60% K 2 O equivalent) selling for $AU395/tonne in 2002 [3] , this fertiliser market is both a high value and high volume market, with the potential to consume significant quantities of fly ash. Studies using natural zeolites have demonstrated significant improvements in fertiliser efficiency for zeolites compared to soluble salts (Clinoptilolite is 7 – 9 times more efficient than KNO 3 in potting medium [4] ). This paper examines the hydrothermal process for producing zeolites (such as Analcime, Cancrinite, Chabazite, Gismodine, and Gmelinite) from coal fly ash, including the relationship between zeolite types produced and operating conditions, desirable zeolite properties for controlled release fertilisers, optimal production conditions, and economic implications. 1 2005 World of Coal Ash (WOCA), April 11-15, 2005, Lexington, Kentucky, USA http://www.flyash.info

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Controlled Release Zeolite Fertilisers: A Value Added Product Produced from Fly Ash

Alex D. Elliot and Dong-ke Zhang Cooperative Research Centre for Coal in Sustainable Development, Centre for Fuels and Energy, Curtin University of Technology, GPO Box U1987, Perth, WA 6845 AUSTRALIA KEYWORDS: Zeolite, Agriculture, Fertiliser, Coal Fly Ash ABSTRACT Of 13 million tones of coal ash produced in Australia in 2003, only 34% is used in some way, with only 15% utilised in applications of value[1], while the remainder is accumulated in landfills and ash dams. This low level of ash utilisation is inevitable due to the combination of inherently high transport costs, and relatively low value products. This situation argues for more value-added utilisation of coal ash to overcome the transport cost barrier. Zeolite synthesised from fly ash for agricultural application as a controlled release fertiliser, is a technology which offers considerable advantages in terms of economic, technical and environmental performance. The Australian fertiliser market consumed 192 Kt of potassium (K) in 1999[2]. Assuming a cation exchange capacity of 3.5, this market is equivalent to 1.5 Mt of zeolite per annum, requiring roughly an equivalent amount of fly ash to produce. With Muriate of Potash (KCl, 60% K2O equivalent) selling for $AU395/tonne in 2002[3], this fertiliser market is both a high value and high volume market, with the potential to consume significant quantities of fly ash. Studies using natural zeolites have demonstrated significant improvements in fertiliser efficiency for zeolites compared to soluble salts (Clinoptilolite is 7 – 9 times more efficient than KNO3 in potting medium[4]). This paper examines the hydrothermal process for producing zeolites (such as Analcime, Cancrinite, Chabazite, Gismodine, and Gmelinite) from coal fly ash, including the relationship between zeolite types produced and operating conditions, desirable zeolite properties for controlled release fertilisers, optimal production conditions, and economic implications.

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2005 World of Coal Ash (WOCA), April 11-15, 2005, Lexington, Kentucky, USA http://www.flyash.info

1 BACKGROUND The utilisation of fly ash from coal-fired power plants is at present realised by low technology applications such as in cement, and construction materials (including mine backfills, soil stabilisation, engineered fills, and road base). However, ca. 500 million tonnes of coal ash is produced globally each year, with only 20% effectively utilised[5], with 13 million tonnes of coal ash currently produced in Australia each year, of which 34% was effectively utilised, and only 15% utilised in applications of value in 2003[1] see Figure 1. The remainder is accumulated in landfills and ash dams, causing considerable adverse environmental effects and public concerns. This low level of ash utilisation is inevitable due to the inherent high transport costs for the relatively low value products produced, especially in Australia where a small population is spread over a large area. This makes it difficult for ash to compete with alternative raw materials. Low value beneficial strategies have proven capable of utilising significant quantities of fly ash in the short term, but incapable of maintaining this in the long term. The cement market is close to saturation[6], it will continue to grow, but is incapable of any major increase in its use of fly ash. This situation argues for the increased diversification of exploited utilisation strategies, in particular, more value-added high technology utilisation of fly ash as an effective means to overcome the transport cost barrier. Zeolite synthesised from fly ash is a technology, which offers considerable advantages in terms of economic, technical and environmental performance.

Figure 1 : Current state of ash utilisation within Australia; (i) Statistical trends grouped according to value, reproduced

from the ADAA[1], (ii) Strategies available for exploitation have been classified into groups according to their usefulness and processing intensity*

* For non-beneficial use, fly ash is seen as having no value, and is generally an economic burden to the ash producer, eg landfill. In simple utilisation strategies, fly ash is the final product, eg agricultural products, or is blended to form the final product, eg blended cement. In advanced utilisation strategies, fly ash is processed to produce a final product, eg zeolites, or to extract a product from the ash, eg cenospheres.

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A significant number of zeolite markets have been identified and classified into groups, see Figure 2 below. Of these markets, agricultural applications offer competitive prices (see Figure 3 below), and have the largest potential market volume for zeolites.

Figure 2: Classification of different zeolite markets

Figure 3: Value of natural zeolites from various sources in both Australia, and the United States of America* where [1]

General range in US natural zeolite prices[7] [2] Price range for -425 µm clinoptilolite in the US[8] [3] Price range for -425 µm chabazite in the US[8] [4] +425 µm zeolites for industrial and agricultural applications in the US[9] [5] -425 µm to -43.2 µm zeolites for industrial and agricultural applications in the US[9] [6] Average sale prices for horticultural zeolites by Talon

resources in Australia, based on information in[10] 1.1 What is a Zeolite? Zeolites are alumino-silicate minerals with a structure containing SiO4 and AlO4

− tetrahedra, linked together with adjacent tetrahedra sharing oxygen to form distinctive crystalline structures (framework structures), contain large vacant spaces (cages) (see Figure 4 below) that can accommodate cations (Na+, K+, Ba2+, Ca2+), as well as large molecules and cation groups (H2O, NH4

+). The cations are distributed throughout the material and play a charge balancing role with the AlO4

− tetrahedra. The cage structures of zeolites are interconnected in three dimensions by channels of constant diameter (see Figure 4 below). Only molecules with a small enough size to pass through channels can enter the internal structure of the zeolite (geometrical selectivity). These void spaces can be filled with water (or other molecules), which can be driven off and reabsorbed without changing the framework structure. * Assuming an exchange rate of $AU1 = $US0.75

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Ca2+

Figure 4 : Illustration of SiO4 and AlO4

− tetrahedra, the orientation of tetrahedra to form framework structures*, and external access to internal framework structure through channels

The charge balancing cations (Na+, K+, Ba2+, Ca2+) can be exchanged with other cations in aqueous solution, without affecting the aluminosilicate framework (see ion exchange capacity in Table 1 below).

Table 1: Typical properties of some economic zeolites, adapted from Holmes (1994)[9]

Zeolite Typical unit cell formula Framework structure[A]

Void volume

(%)

Specific gravity

Channel dimensions

(Å)

Cation Exchange Capacity[B]

(meq/g) Analcime Na16(Al16Si32O96).16H2O ANA 18 2.24-2.29 2.6 4.54 Chabazite (Na2Ca)6(Al12Si24O72).40H2O CHA 47 2.05-2.10 3.7×4.2 3.81 Clinoptilolite (Na4K4)(Al8Si40O96).24H2O HEU 39? 2.16 3.9×5.4 2.54 Erionite (Na2Ca6K)9(Al9Si27O27).27H2O ERI 35 2.02-2.08 3.6×5.2 3.12 Laumontite Ca4(Al8Si16O48).16H2O LAU 34 2.20-2.30 4.6×6.3 4.25 Mordenite Na8(Al8Si40O96).24H2O MOR 28 2.12-2.15 2.9×5.7 2.29 Linde A[C] Na12(Al12Si12O48).27H2O LTA 47 1.99 4.2 5.48 Linde X[C] Na66(Al66Si106O364).264H2O FAU 50 1.93 7.4 4.73

[A] Framework structure types are classified according to a capitalised three letter code [B] Calculated from unit cell formula, [C] synthetic phases

Different zeolites have different selectivity for different cations, a strong selectivity for a particular ion will see that ion preferentially exchanged into the zeolite, and the non-selective species preferentially released into solution. These properties distinguish zeolites as a unique material and are important in its application. For many applications, like controlled release fertilisers, most zeolites will suffice. However not all zeolites were created equal, consequently some zeolites perform better than others for a particular application. Understanding this and knowing how to control the production process to produce the desired zeolite product is critical for the identification and manufacture of a superior product.

* The framework structure graphic was created using chabazite crystal data[11] and Balls & Sticks[12] 3D chemical visualisation software

K+

NH2-CO-NH2 K+

- - +

+ + -

- +

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1.2 Cation Exchange and Fertilisers Cation exchange is a reversible chemical reaction between cations (eg plant nutrients K+, NH4

+, Ca2+, Mg2+) in the solid phase (eg zeolite) and in solution (eg water in soil). The behaviour of this exchange will depend upon the selectivity of the zeolite. For a non-selective zeolite, all nutrients are exchanged into the lattice and are released to maintain the same cation ratios in solution as in the zeolite. A dynamic equilibrium occurs where the zeolite behaves as a general ion buffer. However by the nature of ion exchange reactions, ion exchangers always exhibit a greater selectivity or affinity for particular ions over others, and have an ordered selectivity sequence for cations [13; 14] Due to differences in pore size and framework charge, different zeolites have different selectivity for different elements. The selectivity sequences for zeolite NaP1 (GIS) produced from fly ash are listed below; however previous work has not included cations important to agricultural applications. The differences in selectivity may be due to slight variations in the Si/Al ratio of the zeolites produced.

Cr3+ > Cu2+ > Ba2+ > Fe2+ > Pb2+ > Zn2+ > Cd2+ > Ni2+ [15]

Pb2+ > Sr2+ > Cu2+ > Cd2+ > Zn2+ > Cd2+ > Cs+ [16]

Ba > Cu > Cd ~ Zn > Co > Ni [17]

Fe3+ = Al3+ ≥ Cu2+ ≥ Pb2+ ≥ Cd2+ = Tl+ > Zn2+ > Mn2+ > Ca2+ = Sr2+ > Mg2+ [18]

Cr3+ > Zn2+ > Pb2+ [19]

Under equilibrium conditions a cation that a zeolite has a high selectivity towards, will have a higher relative concentration (relative to a low selective cation) in that zeolite than in the solution. The relative selectivity between any two cations is usually described by the separation factor ( ), Equation 1, or the selectivity coefficient ( ), Equation 2 below.

BAα B

AK

( ) ( )( ) ( )AqBZA

AqAZBBA XX

XX=α [13; 14] (1)

( ) ( )( ) ( ) AB

BA

ZAqB

ZZA

ZAqA

ZZBB

A XX

XXK = [13; 14] (2)

A zeolite loaded with K+ and/or NH4

+, and applied as a fertiliser onto the soil, will result in the exchange of K+ and NH4

+ ions from the zeolite to water in the soil to maintain an equilibrium concentration in the soil. As K+ and NH4

+ are stripped from the soil by plants, more is released by the zeolite to maintain a dynamic equilibrium between the soil and the zeolite. In conjunction with the release of K+ and NH4

+ ions, other ions in the soil, such as Ca2+ will be exchanged into the zeolite lattice to maintain charge neutrality (See equations 3 and 4). By removing calcium from the soil system, the

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insoluble calcium phosphate is replaced by soluble potassium and ammonium phosphates, making them more available to plants [4]. By having fewer nutrients in solution at any one time, less is lost through leaching, and the efficiency of fertiliser application is improved, see Figure 5 below. By controlling the selectivity of zeolite, the concentration of nutrients in solution can be controlled. Urea is a nitrogen fertiliser that is very soluble in water, and can be absorbed into dry zeolite when molten and crystallised at 132°C. This nitrogen fertiliser’s slow release is controlled by two mechanisms (i) physical encapsulation and (ii) ion exchange of NH4

+ into zeolite. Phosphate rock can be blended with this urea zeolite fertiliser to act as a slow release phosphate fertiliser, whose release is controlled by two reactions (1) ion exchange of calcium into the zeolite, and (2) acid leaching from conversion of ammonium ions to nitrate, producing a fertiliser with slower release than super phosphate [20].

)(22 AqCaKZ ++ ⇋ (3) ++ )(2 2 AqKCaZ

)(2

42 AqCaZNH ++ ⇋ (4) )(42 2 AqNHCaZ ++

Zeolites can be loaded using K+ and NH4

+ salts following the synthesis stage, or by treating a waste stream containing the desired nutrient, for example, NH4

+ and PO43-

can both be removed from solution by substituting NH4+ with Ca2+ in zeolite P which

then results in the precipitation of Ca3(PO4)2 [21], and the spent zeolite can be used as a

soil amendment.

Rain or Irrigation

N, K, P Fertiliser

Lake

Figure 5 : An illustration of nutrient cation exchange between zeolite fertiliser and the soil-water environment

Zeolite

Water Table

River

Crop Sea

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1.3 The Market for Slow Release Fertilisation The Australian fertiliser market is summarised in Table 2 below. The fertiliser market is filled with a variety of different fertiliser types that target different applications; Table 3 below lists a few different potential zeolite controlled release fertilisers, as well as some established fertilisers for comparison. The scale of the fertiliser market in terms of zeolites is illustrated in Table 4 below.

Table 2 : Australian fertiliser consumption in terms of nutrients (tonnes) [2] 1996 1997 1998 1999Nitrogen (N) 762,000 811,900 857,300 1,003,000Phosphorus (P) 478,800 436,700 482,100 432,900Potassium (K) 158,900 203,800 195,600 192,300Total 1,399,700 1,452,400 1,535,000 1,628,200

Table 3 : Fertiliser prices

Element (mass %) Fertiliser N P K Ca

Cost[A]

($AU/tonne) Urea (U) [3] 46 0 0 0 340 Phosphate Rock (P) 0 14 0 35 55 [B]

K-Zeolite (K) [C] 0 0 13.0 0 150 NH4-Zeolite (N) [C] 5.0 0 0 0 150 U-K [D] 7.7 0 10.8 0 182 U-N [D] 11.9 0 0 0 182 U-K-P [E] 5.1 4.7 7.2 11.7 140 U-N-P [E] 7.9 4.7 0 11.7 140 CAN [3] 27 0 0 8.4 370 DAP [3] 18 20 0 0 458 NPK Extra [3] 7.5 16.4 10.3 0 502 Pasture Perfect [3] 12.6 0 19.8 0 273 Murate of Potash [3] 0 0 49.5 0 395

[A] For December 2002. [B] Australia’s phosphate rock requirements have always been predominantly imported [22] therefore the price is controlled by international commodity markets, the price for phosphate rock was $US41 in 2002 [23]. An exchange rate of $AU1 = $US0.75 is assumed. [C] Assuming a cation exchange capacity of 3.5 meq/g for Na loaded zeolite. [D] Urea is added to fill zeolites void space in dehydrated zeolite, taken to be 0.35, and the specific gravity is assumed to be 2.3 for all zeolite. [E] One part (mass) phosphorus rock is added to two parts urea filled zeolite.

Table 4 : Potential Australian fertiliser zeolite market

Product Market Demand [A]

(MT / year) Zeolite Value

($AU / T) Market Value

(Millions $AU / year) K-Zeolite [A] 1.5 150 225 Urea-NH4-Zeolite [B] 7.0 150 1050 Blend [C] 7.5 150 1125

[A] Market demand for zeolite component of fertiliser product assuming that it supplies the total 1999 market requirements for potassium. [B] Market demand for zeolite component of fertiliser product assuming that it supplies the total 1999 market requirements for nitrogen. [C] Blend of U-K and U-N such that N and K markets are satisfied, occurs with (20% U-K and 80% U-N)

Based upon information given by IFA et al. (1999)[24], the total world use of nitrogen (N) and potassium (K2O) in fertilisers for 1997, is roughly 70 Mt, and 20 Mt respectively, and represents a significant large potential export market for fly ash zeolites. 1.4 Zeolite Synthesis from Coal Fly Ash Many authors have investigated the hydrothermal synthesis (where the zeolitisation of aluminium and silicon takes place in water at elevated temperatures) of zeolite materials from coal fly ash as the starting material, for direct hydrothermal synthesis[15; 16; 25-45], for

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hydrothermal synthesis using silica extracts[32; 46-48], and for hydrothermal synthesis from alkali fused fly ash[32; 49-53]. Direct hydrothermal synthesis of fly ash is where the ash in its solid particulate form, without any prior chemical transformation, is combined with other reactants and treated hydro-thermally to produce a zeolitic product which contains both zeolite and un-reacted fly ash phases, and is the synthesis method employed in this paper. A number of different types of zeolite have been synthesised from fly ash by previous authors using direct hydrothermal conversion with sodium hydroxide:

• FAU - Faujasite[15; 34], Zeolite X[31; 35; 37; 40; 54], Zeolite Y[54] • LTA - Linde Type A[15; 33-35] • GIS - Zelolite P[16; 21; 25; 26; 31-34; 36], Zeolite P1[15; 17; 27; 30; 37; 38; 40; 43-45; 47; 54], Gobbinsite[44] • ANA - Analcime[15; 27; 30; 36; 38; 43; 54] • CHA - Herschelite[15] • SOD - Sodalite[32; 54], Hydroxy-Sodalite[15; 16; 25; 26; 31; 36; 38; 43; 45], Sodalite Octahydrate[40] • CAN - Cancrinite[36], Hydroxy-Cancrinite[15; 38; 43] • GME - Gmelinite[27] • JBW - Nepheline hydrate[27; 38] • PHI - Phillipsite[40]

1.4.1 Mechanism The mechanism for zeolite synthesis from fly ash in a batch hydrothermal synthesis process has three stages; 1) The dissolution of aluminium and silicon from fly ash, 2) The deposition of aluminosilicate gel on ash surface, 3) The crystallisation of zeolite from aluminosilicate gel[25; 26], see Figure 6 Below. There are three phases in fly ash from which the aluminium and silicon come from 1) Amorphous aluminosilicate glass, 2) quartz, and 3) mullite. The aluminosilicate glass phase is the largest and most unstable of these phases in the hydrothermal environment, and therefore has the highest rate of dissolution[15; 25-29], and is therefore the largest contributor to the zeolites produced. Quartz is less stable than mullite[15; 16;

25-27; 29; 30], although quartz and mullite phases are significantly more stable than the glass phase to the extent that some authors report little to no reactivity[31] for quartz, and[32; 33] for mullite.

Zeolite Fly Ash Particle

Non-reacted fly ash

Figure 6 : Reaction mechanism for the batch hydrothermal conversion of fly ash to zeolite [25; 26]

Reactant Dissolution Crystal Growth

Dissolved Al and Si

Alumino-silicate hydrogel

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1.4.2 Factors influencing zeolitisation There are a number of different factors which influence the zeolitisation of fly ash, they include fly ash properties, alkalinity, temperature, time, cations, templates, water, ageing and seeding. Generally but not exclusively, direct hydrothermal synthesis is carried out between 70 and 200°C, 3 and 48 hours, 0.5 and 5 mol OH− per litre of solution, 2 and 20mL of solution per g of coal fly ash, and usually with Na+ or K+ cations in the form of hydroxides.

1.4.2.1 Coal Fly Ash (Aluminosilicate source) As the source for both silicon and aluminium for zeolite synthesis, the properties of fly ash will have a significant influence on the composition and quantity of these species liberated into solution and therefore the composition of the zeolite produced. The important components of fly ash are 1) Amorphous aluminosilicate glass, 2) Quartz, 3) Mullite, 4) Metal oxides (eg Na2O, K2O, CaO), 5) Iron Oxides (eg Hematite and Magnetite), 6) Unburned Carbon, and 7) Trace elements. Highest synthesis yields >80wt% correspond to highest glass containing fly ash, the lower conversions (yields) are attributed to (i) larger contents of non-reactive phases (hematite, magnetite) in fly ash, and (ii) larger content of resistant alumina-silicate phases (mullite, quartz)[15] The crystallisation of zeolites can be split into primary and secondary crystallisation stages, where the primary crystallisation is fuelled by the amorphous aluminosilicate glass, while the secondary crystallisation is fuelled by the aqueous silica excess left by primary crystallisation and further extracts from quartz and mullite[27]. As the largest and most reactive aluminosilicate source, the glass phase has the greatest influence over the composition and type of zeolite produced. Fly ashes with similar bulk SiO2/Al2O3 ratios produced different zeolites under the same conditions, this is attributed to differences in the composition of glass matrix[15; 27]. Generally higher temperature and higher Si/Al molar ratios favour zeolite P formation, lower Si/Al and temperature favour faujasite and zeolite A[34; 35]. The hydrothermal product has significantly reduced Si content relative to fly ash[36], Tanaka et al. (2002) calculated that only 24% of the Si eluted from fly ash was converted to Zeolite NaP1, with the remainder remaining in the effluent[35]. This excess Si can be utilised by the addition of easily mobilised Al materials. Many metal oxides are readily dissolved in hydrothermal media to produce cations in solution; the effect of charged species in solution is discussed later. Of particular interest is the effect of calcium content in fly ash material on the synthesis of zeolites, materials with a calcium content exceeding a critical level (3-5 wt% of fly ash), have a suppressing effect on the crystallisation of zeolites (in particular zeolite A)[55]. The magnetite phase is not altered during the course of hydrothermal reaction[27; 30]. The iron oxide and unburned carbon phases, which are not altered during hydrothermal synthesis, form part of the zeolitic product; their presence in fly ash makes it impossible to produce a pure zeolite product.

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Most anion forming elements are mobile and largely extracted into aqueous phase during hydrothermal synthesis[17], including As, Se[17], Mo[17; 37], B, and P[37]. Cations forming heavy metals (eg Cu, Pb, Zn) remain in zeolite product (i) as precipitated hydroxides or (ii) absorbed into the zeolite framework[17].

1.4.2.2 Alkalinity The OH− ions create an environment which mobilise Si and Al oxides into solution, OH− ions control the concentrations of saturation and supersaturation which are a key to nucleation and crystal growth[56]. The strength of this base is related to the cations it is coupled with, NaOH has a greater efficiency for dissolving quartz and Mullite than KOH[27], and NaOH solutions produce greater yields of zeolite than KOH solution[15]. The use of NH4OH as base resulted in no change in the CEC compared to fly ash starting material[37]. Increased pH will increase reactant concentrations in solution; this will accelerate the rate of crystallisation[56]. Dissolution is low at low OH− concentration, and results in low zeolite yields[25; 26]. Increasing NaOH concentration result in increased Mullite digestion[27], however increasing the NaOH concentration results in a greater increase in Si dissolution than the increase in Al dissolution from fly ash[29; 33], resulting in a increase in the Si/Al ratio in solution which influences the gel composition and subsequently zeolite product type produced[33].

1.4.2.3 Temperature and time The first phase to crystallise from solution according to Ostwald’s rule of successive phase transformation is the thermodynamically least favourable phase, with time this phase will be followed successively with increasingly more stable phases[56], while increasing temperature increases the nucleation and growth rate of zeolite crystals[56]. Increasing temperature results in increased mullite digestion[27], however it also results in a greater increase in Si dissolution than it increases Al dissolution from fly ash[29; 33], resulting in an increase in the Si/Al ratio in solution which influences the gel composition and subsequently zeolite product type produced[33]. Generally higher temperature favour zeolite P formation, while lower temperatures favour faujasite and zeolite A[34].

1.4.2.4 Charged molecules and templates Templates are structure directing species which increase the stability of particular aluminosilicate structures during zeolite synthesis depending on the properties of template (size and charge), and so have a significant influence on the structure and composition of zeolites produced[56]. The use of sodium produces FAU, EMT, MAZ, ECR-1, and MOR framework structure types, while potassium produces LTL, ERI, and OFF framework structure types[56].

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In addition to being structure and composition directing species, cations also influence the rate of zeolite synthesis[56]. For a fixed 3 hour reaction time and fixed pH, increasing sodium concentration results in increased zeolite P yield, and an increasing potassium chabazite yield for sodium concentration less than potassium concentration, starting from no potassium chabazite when sodium concentration is zero[25; 26].

1.4.2.5 Solvent Water acts as a template by its interaction with cations, in addition to its action as a solvating and hydrolysing agent[56]. Water molecules also enhance crystal growth by filling void space which stabilises the structures[56]. Increasing the liquid to solid ratio in the hydrothermal synthesis of zeolites from fly ash was found to increased dissolution of mullite, quartz and glassy matrix[15], increase yield[15; 30], and decrease reaction rate[15; 28;

30].

2 MATERIALS AND METHODS In this work, the direct hydrothermal synthesis method was employed to produce a zeolitic product from fly ash, which contains both zeolite and un-reacted fly ash phases using batch hydrothermal reactors. Fly ash and all hydrothermal products were analysed using x-ray diffraction (XRD). The fly ash and a selection of hydrothermal products were characterised using elemental analysis, and scanning electron microscopy (SEM).

2.1 Synthesis The reactors were heated in a convective oven, two at a time, placed with geometric symmetry perpendicular to hot air flow. Reaction temperatures were monitored using a thermocouple attached to reactor surface. The oven is programmed such that the reactors reach reaction temperature over a 2 hour period, as illustrated in Figure 7 below. On completion of reaction period, reactors cool convectively in ambient conditions until surface temperature ~70°C (~50 min), at which point they are opened and the contents vacuum filtered hot, and washed with double distilled water.

T1

T1 = Oven temperature set point (ramp) T2

T3 T2 = Oven temperature set point (reaction)

T3 = Reactor surface temperature (reaction)

Tem

pera

ture

(o C)

t1 = Ramp time

t2 = Reaction time

t3 = Cooling time

t1 t2 t3 Time (hours) Figure 7 : Heating profile for batch hydrothermal synthesis experiments

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There are a number of different reaction conditions important to the direct hydrothermal synthesis of zeolite from fly ash, including:

• Reaction Temperature • Reaction Time • n (Si/Al ratio, related to fly ash composition) • M (Cation type) • pH (related to x and p) • p (H2O/Al ratio) • m (excess M+/Al ratio)

The terms in bold are defined by the reaction mixture equation, Equation 5, which is based on 1 mole of Al.

( )( ) OpHnSiOOAlNOOHMzm

zxxz

223213 .5.01

−−−+ +

+ (5)

Nitrate was chosen as the anion to accompany Na+ in the experiments to differentiate Na+ from OH− in NaOH, over more traditional anions (Cl−, CO3

2−) because: • It has highest solubility which increases with temperature • It has a neutral pH • Cl− has low solubility which decreases with increasing temperature • CO3

2− is divalent with NaCO3− a weak base therefore not independent of pH, and

will have a pH buffering effect, see Equation 6 below

−+ 32 NaCOOH ⇋ (6) −+− ++ 3HCONaOH Due to the expressed condition of the sponsor for this work, that all information pertaining to specific reaction conditions is withheld, and therefore the reaction conditions used are not given in this paper. 2.2 Quantitative X-ray diffraction X-ray powder diffraction patterns are produced from the diffraction of x-rays by the different crystal planes within the mineral. Different minerals have different unit cell (smallest divisible unit of crystal structure which is repeated to make the whole mineral) structure, and therefore diffract differently. The pattern that is produced is unique to the mineral from which it was produced (a mineralogical fingerprint). Therefore this technique allows for the qualitative identification of the phases present in a sample. The peak intensities of a phase in XRD are related to the volume fraction of that phase in the sample. The value of peak intensities can be used as a semi-quantitative measurement. Two major limitations with this technique are:

• Uncertainty relating to particle counting statistics • Intensity attenuation/adsorption differences between samples

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By adding an internal standard (eg corundum), the peak intensity of each phase relative to corundum (I/Ic, reference intensity ratio) can be determined, overcoming the second problem above. This technique is considered to be quantitative as composition of phase in sample can be calculated; however the error can be significant. These techniques are not suitable where excessive peak overlap occurs, or where quantitative information on trace phases is required. The diffraction pattern of any known mineral phase can be modelled from theory (using unit cell and atomic position information), by using full profile Rietveld analysis, which fits model to experimental data (for all peaks over whole experimental 2θ range) by refining instrument, microstructural, and atomic parameters, allowing the quantitative determination of the mineralogical phases present in the material (including trace phases) with significantly improved error relative to other techniques. The Rietveld method was chosen for this work because:

• Quantitative information for trace phases was desired • Significant (sometimes total) peak overlap is present in samples • Best confidence in the reliability of result is desired

For XRD experiments, solid samples were ground to finely divided form and standardised samples prepared by combining a measured quantity of sample with a measured quantity of corundum with thorough mixing. The analysis was conducted on a Siemens D500 diffractometer using Cu kα wavelength, with 1° divergence slits, over the range 3 – 70° 2θ, with a 0.02° step size, and a speed of 0.6° min-1. The resulting diffract-o-grams* were quantitatively analysed using multi-component Rietveld refinement method[57] with the program Rietica[58] over the 2θ range 15 – 70°, because at 2θ below 15°, the beam footprint exceeds the bounds of the sample holder resulting in reduced incident intensity to the sample, and therefore peak intensity from the sample. Mineral phases used in quantitative analysis include Corundum[59] (ICSD #73725)†, Quartz[60] (ICSD #200722), Mullite[61] (ICSD #100805), Hercynite[62] (ICSD #86576), Magnetite[63] (ICSD #82237), Hematite[64] (ICSD #82135), Analcime[65] (ICSD #87555), Cancrinite[66] (ICSD #63719)‡, Chabazite[11] (ICSD #18197)§, Zeolite NaP1[67] (ICSD #9950), Gmelinite[68] (ICSD #31240), and Sodalite[69] (ICSD #56710)**.

* Due to unsatisfactory capacity of Rietica to model the amorphous backgrounds present in samples, the backgrounds were fit and removed separately using manual peak removal and background modelling in a spreadsheet † Inorganic Crystal Structure Database (ICSD) ‡ Na1 and H O site occupancies refined 2§ Atomic positions were refined, and Ca atom occupying 12i site was replaced with a Na atom occupying 6h site ** Atomic positions for Na were refined, and a H2O molecule was added to occupy 8e site

13

2.3 Elemental Analysis Elemental analysis was conducted on fly ash and selected hydrothermal products, HTP1 to HTP4, using inductively coupled plasma mass spectrometry (ICP-MS), and atomic emission spectroscopy (AES).

2.4 SEM For SEM analysis, solid fly ash and HTP1 samples were set into an epoxy resin and polished to give a smooth cross section of particles for analysis. SEM analysis included the use of EDS* and EMPA† point analysis for the determination of local chemical composition.

3 RESULTS AND DISCUSSION

3.1 Morphology and Composition To evaluate the chemical and mineral variation within and between fly ash particles, SEM was used in conjunction with EPMA analysis, with the mineralogy inferred from the chemistry. A selection of sample points from a range of particle types is illustrated in Figure 8 below for fly ash, with the chemistry of test points given in Table 5.

Figure 8 : SEM images of fly ash cross sections illustrating the location of EPMA point analysis

AC

B

ED Quartz F

N

H

P Q

Quartz

IG

L

J

O

M

R

K

* Philips XL30 SEM with Oxford Instruments EDS (SiLi detector with super ultra thin window) using INCA Suit version 4.00 software † JEOL JXA-8600 Superprobe with data interpreted by JEOL XM-86PAC quantitative analysis program

14

It can be seen that there is significant chemical and mineralogical variation between different ash particles and even within a single particle. From Table 7 we know that the iron content is high, and the content of iron in mineral phases is low (see Table 8 below), therefore significant iron must be present in the glass phase, this is supported by the EPMA analysis presented in Table 5, where all the glass phases tested contained at leased 4% iron.

Table 5: Chemical composition of phases analysed using EPMA point analysis as illustrated in Figure 8 above (wt%) Phase[A] Point SiO2 Al2O3 Fe2O3 CaO MgO N 2O TiO2 P2O Tot a2O K 5

A 55.2 37.9 4.27 0.59 0.74 1.97 1.18 0.61 102.7 B 59.0 36.3 4.06 0.64 1.37 0.95 103.1 C 60.4 25.9 5.37 0.93 2.23 97.7

54.0 35.2 6.64 0.52 1.72 0.86 0.98 100.9 46.9 33.0 13.0 2.09 1.84 0.76 1.44 1.5 101.0

F 47.4 33.2 6.77 0.65 0.65 1.37 0.57 1.85 93.0 G 57.6 35.8 4.17 1.96 0.77 1.68 102.8 H 51.2 35.1 9.11 0.65 0.83 1.07 1.75 0.59 100.8 I 52.0 40.6 7.06 0.51 101.8 J 38.5 31.1 21.1 3.37 1.76 0.87 1.90 99.5 K 57.2 36.0 4.38 0.58 0.52 1.84 0.98 0.87 102.8

Average 52.7 34.6 7.81 2.04 0.98 0.64 1.46 1.21 1.19 100.6

1.77D E

Glass

Std Dev 6.5 3.8 5.16 1.36 0.57 0.11 0.52 0.43 0.56 3.0 L 64.6 19.3 15.4 100.4 Potash

Feldspar M 69.0 19.4 9.52 99.1 Hematite 93.8 N 0.93 0.59 91.5 Hercynite O 8.30 29.5 52.82 3.98 3.18 1.50 5.19 105.0

P 24.8 29.0 16.0 9.09 4.22 5.39 1.81 91.4 Q 15.7 39.0 24.1 5.25 6.92 4.22 2.58 98.8 Unknown R 24.3 28.6 26.3 7.52 3.08 0.96 1.41 92.8

[A] Mineralogy inferred from chemistry [B] The electron beam volume of excitation of glass phase may include mullite phases which grow within the glass phase, giving a composition which is a blend of glass and mullite phases.

By comparison of fly ash starting material and a hydrothermal product HTP1 produced sing batch non-agitated hydu

trothermal reaction, and characterised using SEM, several

rae p le ic i fl

produc the ra ar ize as ic in e en p is a si ug at the bottom of reactor, which is crushed into granules for handlin and lys

• Three types of particle morpholog st hy roth l p ct:1) cro icles which includes co ne f u ac y a

ed ther by or encapsulated within olite phasd 9

2) ngle se du rt w con in few incl u ct h p s h igh geometric al tru ( a

d F s n *)

nsformations are evident (see Figures 9 and 10): • Th small artic s wh h dom nate y ash are not present in hydrothermal

ts, ave ge p ticle s h signif antly creas d, oft the roductngle solid plg ana is

y exi in the d erma rodu Ma -part mpo nts o n-re ted fl ash p rticlesglu toge ze es (see Figures 9(i) an (ii)) Si pha pro ct pa icles hich ta uded n-rea ed flyas article , and ave h ly cryst line s cture see p rticlesA an B in igure 9(iii) a d (iv)

* 9(iv) is a e oa olis d cros section which fractures crystals long l p

Note Figure n imag of a c rsely p he s a crystalanes

15

3) cluded co e f ea fly sh pa ticle e re ) d 1

1) Phases which retain full spherical form (un-reacted fly ash particles, see

rticle where more reactive phases

parsolutio

Figure 9 : Particle cross sections viewed by SEM for HTP1 illustrating different zeolite phases present in hydrothermal

product

Ex mpon nts o non-r cted a r s (se Figu s 9(ian 0(ii))

• From inspection of the light contrast phases (high atomic number, eg high iron), two types are present:

particles C in Figure 9(iii)) 2) Free iron phases which often occur in spherical like clusters indicating

originally part of spherical fly ash pahave etched away (see Figure 10(iv))

Quartz particles show significant signs of internal etching (see Figures 10(i) and (ii)) Ghosts of fly ash partic les remain where glass has been completely etched awayto reveal mullite crystals which retain the overall morphology of original fly ash

ticle (see Figure 10(iii)). This demonstrates that mullite was accessible to n but did not dissolve under the reaction conditions of HTP1

A

B

(ii)(i)

(iii) (iv) AC

16

(ii)(i)

(iii) (iv)

F l

All of the hydrothermal experiments presented were conducted using the non-agitated batch hydrothermal process, which was expected to follow the reaction mechanism illustrated in Figure 6. Figures 9 and 10 confirm this in part, however the zeolite phase has not grown to surround ash particles, encapsulating them and isolating them from dissolution, instead they have grown into the spaces between particles, and joined ash particles (present in close proximity to each other as sediment at the bottom of the non-agitated reactor) together like a glue (see Figure 9(ii)). The difference is likely to be due to the absence of viscous effects in non-agitated systems which are present in agitated systems. This type of growth creates a spongy macro-scale particulate product which allows access for the continued dissolution of fly ash particles with time. A consequence of large (mm scale) product particles with spongy structure is that reactant solution containing NaOH and NaNO3 can be isolated from the wash procedure, and be present in the product. The presence of NaOH and NaNO3 in products is supported by their bulk Na2O contents, see Table 7, which are larger than prescribed by the zeolite phases present, see Table 8. However neither NaOH nor NaNO3 were detected in any XRD patterns. The discrepancy is most likely due to the formation of amorphous alumina-silicate gel, illustrated in Figure 6, which has not crystallised to form any zeolite. Either way additional x-ray amorphous phases contribute to the “other” components in Table 8. Under these circumstances the dissolution of amorphous glass phases in fly ash cannot be investigated directly using quantitative XRD results.

igure 10 : Particle cross sections viewed by SEM for HTP1 illustrating quartz and mullite phases present in hydrothermaproduct

17

The o product particles are too compli to discriminate between the different zeolite phases and hydro-gel, therefore EDS analysis of zeolites was limited to the highly geometric crystalline phases which are clearly identifiable. From fly ash characterisation we know that a significant portion of iron is present in the glass phase of fly ash. However this has not prevented it from being dissolved by the alkaline hydrothermal solution. The iron released by glass dissolution has been incorporated into the zeolite phases produced by hydrothermal reaction (see Table 6), although it is unknown how it is incorporated (within framework, as a charge balancing cation, or discreet nano-particles). Although the fly ash has significant chemical and mineralogical variation, the variation in the composition of zeolite particles (A and B) illustrated in Figure 9(iii) is low, see Table 6. Table 6 : Chemical composition of phases analysed using EDS analysis of zeolite phases of type A and B as illustrated in

Figure 9 above (wt%) Phase[A] SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 P2O5 H2O Tot

macr cated

Average 56.3 18.4 5.09 0.15 0.03 12.1 0.09 0.11 0.11 8.11 100.6 A Standard Deviation 1.6 0.6 0.50 0.26 0.09 0.7 0.08 0.07 0.17 1.04 2.9 Average 56.3 22.6 2.29 0.48 0.00 10.2 2.14 0.56 0.05 5.17 99.9 B Standard Deviation 2.5 1.1 0.42 0.17 0.01 1.9 1.40 0.13 0.05 1.19 3.2

[A] Compositions based on 14 analysis points for phase A, and 10 for phase B

During the hydrothermal proce resent in solid product, while others present in aqueous reactants form part of the solid

mpletely from the glass phase, and yields are 0.45, 0.19, and 0.12 grams of eolite per gram of fly ash reactant respectively.

ss some elements from the fly ash are leached and notpproduct (eg. Na, H2O). Iron has a very low solubility under hydrothermal reaction conditions, therefore all iron inputted from the fly ash is expected to be present in the hydrothermal product. If Fe2O3 is used as a tracer, changes in the total Al and Si content and in the quartz, mullite and zeolite contents can be quantified. By using Fe2O3 as a tracer (see Table 9), we see that there is a general reduction in the SiO2 content of hydrothermal products from that in the fly ash, while the Al2O3 content remains roughly the same. For HTP1, HTP2, and HTP4, the source of Al in zeolite is almost coz

18

Table 7 : Bulk chemical analysis of fly ash and a selection of hydrothermal products (wt%) Component Fly Ash[A] Fly Ash HTP1 HTP2 HTP3 HTP4

SiO2 53.7 54.8 38.3 39.4 32.7 42.8 Al2O3 23.7 21.9 23.1 23.4 21.9 19.3 Fe2O3 14.5 14.6 14.3 14.6 13.7 13.4 CaO 1.7 1.78 1.80 1.80 1.72 1.57 MgO 1.1 1.11 1.16 1.18 1.Na O 0.27 0.30 6.93 9.57 16.2

11 0.98 2 7.47

88.9 93.5 89.7 89.3

K2O 0.94 0.95 0.46 0.29 0.05 0.61 TiO2 1.4 1.42 1.42 1.43 1.32 1.32

Mn3O4 0.14 0.13 0.13 0.13 0.13 0.12 P2O5 1.7 1.62 0.57 0.94 0.09 1.05 SO3 0.09 0.22 0.03 0.04 0.04 0.10 SrO 0.34 0.31 0.33 0.33 0.31 0.29 BaO 0.48 0.41 0.44 0.45 0.41 0.38 Total 100.1 99.5

[A] From XRF

Table 8 : Bulk mineral analysis of fly ash and a selection of hydrothermal products (wt%)

Component Fly Ash HTP1 HTP2 HTP3 HTP4 Quartz 13.8 3.8 5.7 1.4 8.2 Mullite 11.0 13 11 4.0 9.2

Hercynite 4.6 4.1 4.8 4.7 6.3 Magne

emtite 1. 1

ati 0. 0.7 n ancrin 42 ha

Sodalite 13 8.5 1.9

3 2.1 1.5 .8 ? HA

te 9 ? 0.7 ? alcime 1.4

CC

ite 6.1 1.9bazite 39 5.8

Gmelinite 3.3

Zeolite NaP1 1.0 Other[A] 68.4 33 57 37 66

[A] x-ray amorphous phases and mineral phases not quantified (indicated by ?) due to 8 phase limitation of Rietica and excessive peak overlap of trace iron phase peaks with major zeolite phases

Table 9 : Ratio of components composition (wt%) to that of Fe2O3 Tracer (wt%) [A]

Component Fly Ash HTP1 HTP2 HTP3 HTP4 SiO2 3.70 2.68 2.70 2.38 3.20 Al2O3 1.63 1.61 1.61 1.60 1.44

Quartz 1.95 0.26 0.39 0.10 0.62 Mullite 0.76 0.90 0.78 0.29 0.69

Zeolites --- 3.07 1.33 3.66 0.80 [A] Multiplying this ratio by grams of Fe2O3 per gram of fly ash (0.145) gives the yield in grams of component per gram of fly ash reactant

rom analysis of trace element concentrations F

foin fly ash and hydrothermal products

llowing various hydrothermal conditions (see Table 10 below), showed some elements were significantly reduced in concentration (As and Mo), some were slightly reduced (Hg, Pb, Cu, V), some stayed constant (Cr, Ni, Zn), and some were already at or below the detection limit of analysis (B, Cd, Se). Most of the elements of greatest concern have been reduced in concentration to or below their average levels in soil, shale, or crust, and not likely to present a problem for agricultural application.

19

T C Fly HTP1 HTP3 4

able 10 : Trace element analysis for fly ash and a selection of hydrothermal products (ppm) [70] 0]Element[A] Soil Shale[7 rust[70] Ash HTP2 HTP

Elements of greate cern st conArs As 7 6 <1.0 <1.0 Boron B 30 20 20 40 0 Cad Cd 0.6 0.5 0.5 <0.5 5 Mercury Hg 0.1 0.0 0.09 0.06 4 Mo Mo 1 12 2 1.5 Lead Pb 20 52 41 30 8 Selenium Se 0.4 <5 <5 <5 5 Elements of Moderate Concern Chromium Cr 55 150 150 150 0 Cop Cu 25 82 76 55 7 Nickel Ni 20 254 256 258 0 Van V 80 14 68 36 4 Zinc Zn 187 189 180 7

enic 13 1 <1.0 1 130 10 20 2

mium 0.3 0.2 0.5 0.0.18 0.08 7 0.05 0.0

lybdenum 2.6 1.5 .5 2.5 2 25 13 49 30.5 0.05 <5 <

90 100 200 15per 40 55 79 6

68 75 270 24adium 130 135 6 98 8

70 120 70 196 15[A] Classified according to environmental concern as presented by Clarke and Sloss (1992)

3.2 Alkalinity (pA number of zeolite products were produced f fly ash us non-agitated batch reactor over a range of pH conditions. The diffraction pattern for fly ash A and three of these products is presented in Figure 11, from which the follow bservation n be no

emergen of zeolite p asing ze ite peak inten y with increasing reaction soup pH ft in zeol s produced sodalite to ancrinite with creasing pHcrease in he quartz pea with increas pH

How com rison due to

1) at 2θ of 13.9, 18.8, 24.2 and 27.4. The first and third of these peaks corresponds to the 011 and 11 ond to the 110 0 hkl o ancrinite respectively. Rietveld refinement is needed to de-convolute these peaks. From the quantitative results (see Figures 12 and 13) we see

consum of quartz mullite wit easing reactant pH emerge of zeolite ses canc and sod and the ative

[71]

H)

rom ing

ing o s a cted: • The ce eaks• Incre ol sit• A shi ite from c in • A de t k ing

ever there are limitations with direct peak pa peak overlap between sodalite and cancrinite products. There are 4 major peaks which emerge (see Figure 1

2 hkl orientations of sodalite, while all four peaks corresp11 030 121 rientations of c

: • The ption and h incr• The nce pha rinite alite ir rel

abundance as a function of pH

20

Figure 11 : XRD patterns of fly ash A and hydrothermal products of fixed m and increasing pH, as well as individual

sodalite and cancrinite zeolite phases.

Figure 12 : Mineral compositional trends for hydrothermal products as a function of reaction pH (i) m = constant, (ii) X = 1

Figure 13 : Component composition expressed as a fraction of original component composition in fly ash (wt%/wt%)*

* Note: If fraction > 1 then the component is present in a greater concentration in hydrothermal product than in fly ash reactant. For example, between first two pH conditions where glass is dissolving

(i) (ii)

21

From Figure 12(i), where m is fixed and pH is varied, the yield and type of zeolite produced by hydrothermal reaction changes. By comparison, from Figure 12(ii), the same pH conditions are investigated with x = 1 therefore different m. The abundances and types of zeolite produced are different to the fixed m conditions. Clearly M+ and OH¯ play a different role in the overall reaction mechanism. Previously Murayama, et al. (2002)[25; 26] investigated the substitution of NaOH and KOH for Na2CO3 wile maintaining a constant concentration of M in solution, and concluded that:

• The OH− in alkali solution contributes to the dissolution of coal fly ash, while • The Na+ contributes to the crystallisation of zeolite P

Building on this, it is proposed that:

2. M controls the type rate of zeolite crystallisation from the amorphous alumina-silicate gel

trations will change the saturation curves (solubility) of species that participate in the hydrothermal reaction, therefore the reaction chemistry.

When m is a constant and pH is varied, there is a general increase in the dissolution of quartz and mullite with increased pH, which supports point 1 above. Under constant pH conditions, if m is varied, different types and quantities of zeolite are produced, which supports point 2 above.

phases remains close to 1 while mullite which is encapsulated within glass phase is significantly consumed, indicating the significant presence of an amorphous alumina-silicate gel. The composition of amorphous phase under condition m = constant and pH ≥ HTP2 is substantially lower than under condition x = 1, which is maintained as x approaches 1 for m = constant (solubility differences narrow), indicating that increasing m increases the rate of zeolite formation out of the amorphous alumina-silica gel precursor, which supports point 2 above. Under condition x = 1, substantial quartz dissolution starts at a lower pH than substantial mullite dissolution occurs. Under condition m = constant, this quartz and mullite dissolution is shifted up to a higher pH. The presence of increased NaNO3 has decreased the solubility and therefore dissolution of quartz and mullite, which supports point 3 above.

1. OH− controls the dissolution rates of Al and Si sources while + of zeolite product, and the

3. Together MOH and MNO3 occupy soluble space, and the selection of different compounds and concen

Ufor iron and x-ray amorphous

nder condition x = 1 and pH ≥ HTP2, the fraction of original component composition

significantly faster than quartz or mullite and no zeolites are forming, products have glass and mullite composition fractions > 1

22

3.3 Reaction time

ation or changing solution hemistry with time. Increased ti ees the continued dissolution of quartz phase

Figure 1 r hydrothermal products as a function of time (i) Wt%, (ii) Component tion of original component composition in fly ash (wt%/wt%)

nder the hydrothermal conditions of constant pH and x = 1, the concentration of NaOH

reasing p, while the concentration of zeolites ecreases (see Figure 15). This is due to increased fly ash reactant, with decreased p,

te not produced t the higher p’s (see Figure 15). This is likely to be due to either or both:

s of zeolite shifts towards the more reactive particles. As a consequence the solution Si/Al ratio may also change

Results from a study of reaction time on hydrothermal synthesis for conditions of constant pH, p, m and T are presented in Figure 14 below. Following an initial short reaction time, chabazite is the only zeolite product. Increased reaction time sees the emergence of new zeolite phases (Gmelinite, Analcime, and Zeolite NaP1), which agrees with Ostwald’s rule of successive phase transformc me stowards an asymptotic limit (probably due to decreasing pH with increased zeolite production as OH− is consumed).

4 : Mineral compositional trends fo

composition expressed as a frac

(i) (ii)

3.4 Water Content (p) Uin initial reaction solution is constant for all p. The concentration of quartz and mullite reactants in products increases with decdthat requires dissolution, and does not have sufficient time to dissolve and react. The concentration of zeolite product at HTP4 is 11% while it is 32% for the highest p presented, however HTP4 contained 4 times as much fly ash reactant. The total zeolite yield has increased; this is likely to be due to the presence of 4 times as much of the more reactive reactants in HTP4. The condition HTP4 sees the emergence of Zeolite NaP1 and chabazia

1. The formation of zeolite and its precursor gel strips Na+ from solution, therefore available m decreases with time, and lower m favours the formation of different zeolites as discussed in Section 3.2

2. With decreasing p, the fly ash particles dissolving and contributing to the synthesi

23

ponent composition in fly ash (wt%/wt%)

3.5 Cation Exchange Selectivity The sole source of potassium in HTP1 hydrothermal reaction is from the fly ash. The total mole ratio of sodium to potassium in HTP1 reaction soup was 59. The Na/K mole ratio in phase A and B indicated in Figure 9 and Table 6 previously is 204 and 7 respectively. Clearly there is a difference in cation selectivity between these different zeolite phases, with phase A selective toward sodium (Na > K), and phase B selective toward potassium (K > Na). From this result, if both phases were loaded with potassium (for er oil solutio ill perform ifferently as a fertiliser; therefore different zeolite fertiliser products can be designed

requirements of specific applications.

(coa specaluproper 10

he 30m3 reactor cost $US60,000 in 1987[72]. Using a ChE index of 324 in 1987[73], and an extrapolated (from ChE[74]) index of 404 for 2005, a location factor of 1.3 for Australia[72], an exchange rate of $US0.75 per $AU. Assuming a plant cost factor of 4,

F

(i) (ii)

igure 15 : Mineral compositional trends for hydrothermal products as a function of p (i) Wt%, (ii) Component composition expressed as a fraction of original com

use as a fertiliser), phase A is expected to release potassium to maintain a highn concentration of potassium than phase B. In other words they ws

dand produced to meet the 3.6 Economic Implications To give a basic idea of the economic implications of processing conditions some crude economic calculations are presented below based on reasonable assumptions. Assuming a 300 kilo-tonne per annum zeolite plant, which sells product for $AU150 per tonne of zeolite, has an operating cost of $AU100 per tonne, operates under a 30% corporate tax rate, with a 10 year double declining depreciation period, a 15 year project life span, and a 10% discount rate. For a present value ratio of 1, the fixed capital

vestment is $AU100 million. in With a maximum reactor size of 30m3 [72], and assuming that 80% of this volume is available for solid and liquid reactants, 300 days of 24 hou s continuor us operation

ntinuous reactors), assuming that solids (fly ash, zeolite and sodium hydroxide) have ific gravity of 2. With an appropriate pH, an x of 1 (see Equation 5), and an

minium (Al2O3) composition equal to 25wt% of fly ash, where 1 tonne of zeolite is duced per tonne of fly ash. Then 20 reactors are required per hour of residence time

0 units of p. T

24

and that the reactor section of whole plant cannot exceed 10% of FCI, there is a 19 reactor limit. For a 3 hour residence time, p is limited to 32 which is equivalent to 11.3 g H2O per g Al2O3 in fly ash.

4 SUMMARY AND CONCLUSIONS Zeolite synthesised from fly ash for agricultural application as a controlled release fertiliser is a technology which offers considerable advantages as a fly ash utilisation strategy, in terms of economic, technical and environmental performance because:

• It is a value added product capable of overcoming the transport cost barrier inhibiting many traditional fly ash utilisation strategies, and capable of making use of a substantial portion of the fly ash produced annually in Australia

ency compared to traditional fertilisers resulting in

tive results have been repeated using quantitative XRD

• Dissolution increases with increasing pH

ditions. • Quantitative mineral analysis revealed that the amorphous alumina-silica gel

llisation from the amorphous alumina-silicate gel. •

• It has improved fertiliser efficiincreased productivity, and reduced nutrients being leached into the environment

From experimental results discussed, the following conclusions can be drawn

• The presence of iron in the glass phase of fly ash has not inhibited its dissolution • The dissolution of iron containing glass has resulted in iron containing zeolites • The zeolite phases produced are more homogeneous in composition than the fly

ash used to make them • Different types of zeolite can be produced from fly ash by changing reaction

conditions • Different zeolite phases have different cation exchange selectivity’s therefore will

perform differently as a controlled release fertiliser Established experimental qualitatechnique including:

• The higher reactivity of quartz with OH− than mullite

The advantage of using XRD is that it can quantify changes in the composition of quartz and mullite reactants, and zeolite products with changes in hydrothermal reaction conditions, revealing more information and allowing direct comparisons of the changes taking place with changing con

precursor to zeolite synthesis described in Murayama’s mechanism (see Figure 6) does not completely crystallise to form zeolites, and is present in hydrothermal products.

• By using NaNO3 the influence of m (excess M+), can be studied independent of pH (OH− concentration).

• In addition to controlling the type of zeolite produced, m controls the rate of zeolite crysta

Increasing m at constant pH reduces the solubility of reactants. • By using Fe2O3 as a tracer, total yields of zeolite and total quantities of quartz

and mullite leached per gram of fly ash reactant can be determined.

25

Fly ash zeolitic materials have a reduced overall impurity load as a result of leaching during the synthesis stage resulting in (1) a more environmentally friendly product, and therefore (2) a more attractive product for use in agriculture relative to fly ash.

lian Go n The au e staff at thewith th

rocess Mineralogy, University of Western Sydney for their assistance with the use of

6 E [1]

5 ACKNOWLEDGEMENT The authors gratefully acknowledge the financial and other support received for this research from the Cooperative Research Centre (CRC) for Coal in Sustainable Development (CCSD), which is established and supported under the Austra

ver ment Cooperative Research Centres program.

thors would like to acknowledge the help and support received from th Department of Applied Physics, Curtin University of Technology for their assistance

e use of SEM and XRD facilities, and the staff at the Centre for Industrial & PEPMA facilities.

R FERENCES

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Federation of Australia. http://www.fifa.asn.au/fertilizer_in_aust/stats.html. 11

armers Co-operative

Jan. 2003. [3] UFCC (2002). "Fertiliser price list 2002/03". United F .

Price%20info2.pdfhttp://www.ufcc.com.au/html/fertilisers/5713-UFC%20 . 10 Jan. 2003.

[4] Ming D.W. and Allen E.R. (2001). Use of natural zeolites in agronomy,

horticulture, and environmental soil remediation. Natural zeolites: occurrence, properties, applications. Bish, D.L. and Ming, D.W. Washington, DC, The Mineralogical Society of America. 45: 619-654.

Nairn J., Blackburn D. and Wilson M. (2001). [5] Research and development for fly

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[7] Virta R.L. (2002). Zeolites - 2001. Minerals yearbook: volume 1 -- metals and

minerals. Reston VA, US Geological Survey. 1.

26

[8] Eyde T.H. (2002). "Zeolites: a look at 2001 activities" Mining Engineering 54(8):

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