ZA200801062B - Applications for ash and its derivates - Google Patents

Description

RHIC t12008/ ; y 8

HHI BE 701062

DE J

EE APPLICATIONS FOR ASH AND ITS DERIVATES s FIELD OF INVENTION oo

The present invention relates to applications of fly ash and its derivates.

More particularly, the present invention relates to applications for fly ash and its derivates, including water treatment, mine backfilling, lining and walling.

BACKGROUND TO INVENTION

Many power stations in South Africa are coal dependent and produce nearly 20 ‘megatons of fly ash (FA) per annum. Currently very little (approximately 5%) of that fly ash is utilized properly and the rest is disposed of in ash dams and landfills. Poor management of the fly ash induces various problems including rehabilitation, leaching of toxic elements into the ground water, soil and air pollution, loss of cultivated land etc.

Acid Mine Drainage (AMD) is another environmental hazard that is characterized by a very low pH and high sulphate and heavy metal concentrations that could have a negative effect on the environment. South Africa currently produces nearly 44 mega litres of AMD each year which is estimated to increase to nearly 110 mega litres by 2010. These figures are high and require a treatment approach that would not only neutralize AMD but also effectively manage the waste sludge that emanates as a result of such treatment. :

Fly ash has free alkalinity imparted by CaO and other ash components, as well as a very high surface area and small particle size. These properties provide for a good neutralisation agent and AMD ameliorant.

It is an object of the invention to suggest applications for fly ash and its derivatives, which will assist in overcoming these problems.

SUMMARY OF INVENTION

According to the invention, a method of using fly ash includes the steps of mixing acid mine drainage. with fly ash; and of allowing the acid mine drainage

DrG Ref: 666614 i to react with the fly ash in a neutralisation reaction, to form a solid residue product and pH neutral process water. - The fly ash may consist of at least 70% of the elements SiO,, Al,0; and Fe,0s. - The acid mine drainage and fly ash may be mixed in a ratio of between 1:1 to 40:1 for relatively less contaminated acid mine drainage.

The acid mine drainage and fly ash may be mixed in a ratio of between 1:1 to 10:1 for relatively highly contaminated acid mine drainage.

A zeolite made from fly ash may be contacted with the process water recovered after the neutralisation reaction to remove toxic elements from the process water.

A zeolite made from the solid residue product may be contacted with the process water recovered after the neutralisation reaction to remove toxic elements from the process water.

The toxic elements may include at least one of Fe, Ca, Na, Sr, As, Cd, Ba, Mo.

Also according to the invention, a method of using fly ash includes the steps of mixing acid mine drainage with fly ash in a neutralisation reaction; of extracting a solid residue from the neutralisation reaction; and of using the solid residue in a mine for backfilling operations.

The acid mine-drainage and fly ash may be mixed at ratios between 1:1 to 10:1 in the neutralisation reaction.

The acid mine drainage and fly ash may be mixed at ratios between 4:1 to 7:1 in the neutralisation reaction.

The neutralisation reaction may be stopped after a pH greater than pH 7 is obtained.

The neutralisation reaction may be stopped after a pH greater than pH 9 is obtained. :

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. The neutralisation reaction solution may be allowed to settle for at least one hour to allow separation of the solid residue from the neutralised process water. - The solid residue may be converted into a slurry for transportation to a mine. - The slurry may be formed by mixing the solid residue with either neutralised

S process water or tap water or by partial dewatering of the resultant sludge from the neutralisation reaction.

Further according to the invention, a backfill for a mine includes a solid residue product of a neutralisation reaction between acid mine drainage and fly ash.

The solid residue may be in the form of a slurry.

The backfill may include fly ash and a cement additive added to the solid residue.

Further according to the invention, a method of using fly ash includes the steps of placing the fly ash in a mine void so that it can be contacted and react with acid mine drainage seeping though the mine void.

DESCRIPTION OF THE INVENTION BY WAY OF EXAMPLE

: An integrated acid mine drainage (AMD) management scheme in accordance with the invention using fly ash includes four components. The first component uses fly ash to neutralize AMD and to attenuate toxic element concentrations in

AMD streams. The second component uses solid residues (SR) as a suitable backfill material to stabilize mines. Backfiling of mines is a way to avoid subsidence, provide support to pillars and walls and reduce the void volume.

Backfilling also plays a role in mitigating the environmental concerns of underground fires and the future production of AMD as well as neutralising existing AMD. The third component allows high and low temperature synthesis of zeolites using solid residues or fly ash and use of such zeolites to treat post- neutralization waters. Finally, the fourth component provides a passive treatment option called “Ash walling” in which ash is placed in mine voids as an in situ barrier. The ash barrier neutralizes the AMD that is produced in the mine

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] voids and therefore reduces the threat of surface and ground water contamination. - Neutralization of AMD and Toxic element removal - The fly ash used was either derived from anthracitic or bituminous coal, with the

S characteristic that SiO, + Al,O; + Fe,0; 2 70% as can be seen from Table 1.

Important factors in the neutralization and sulphate reduction of AMD during active treatment with fly ash are: 1. Free CaO content in fly ash; 2. Amount of fly ash in the reaction mixture or FA: AMD ratio used (indirectly amount of CaO + in the mixture); 3. Reaction time; 4. Chemistry of AMD (important parameters being concentration of Fe2+/Fe3+ and Al3+); 5.pH of AMD (must be acidic).The % of CaO in the fly ash, which was 8.43 (Table 1), is very important as it plays a major role in the neutralization of AMD. Table 1 also shows that the fly ash has a number of trace elements in ppm level.

The pH of the AMD used in neutralization experiments was 2.39, indicating that it was highly acidic (Table 1).

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. Table 1: Composition of fly ash and AMD used in neutralization experiments ] Oxides(%w/w) & Fly ash AMD ] element(ppm

SiO, 53.4 pH 2.39

Al,O; 23.4 EC (mS/cm) 10.83 i CaO 8.43 Acidity(mg/L 6950

CaCo0s;)

Fe 0s 4.72 TDS (mg/L) 16765

MgO 2.70 Na (mg/L) 359

TiO, 1.34 Mg (mg/L) 2660

KO 0.49 Al (mg/L) 1070

Na,O 0.35 Si (mg/L) 82.0

MnO 0.06 K (mg/L) 23.0

P.Os 0.35 Ca (mg/L) 653

Cr,0; 0.03 Mn (mg/L) 226

NiO 0.011 Feta (Mg/L) 5600

V,0s 0.019 Fe?* (mg/L) 3725

ZrO, 0.052 Fe?* (mg/L) 1450

LOI 2.36 Ni (mg/L) 6.95

Cu 57.9 Co (mg/L) 4.30

Mo 6.56 Zn (mg/L) 49.0

Ni 58.2 Sr (mg/L) 7.69

Pb 29.1 Mo (mg/L) 0.04

Sr 2056 Ba (mg/L) 0.21

Zn 25.4 S0,* (mg/L) 11890

Zr 536.1 CI" (mg/L) 729

Co 10.4

Cr 122.7 \Y 145.8

Ba 1559.2

The pH of mine water depends on many factors such as the abundance and extent of pyrite weathering, degree of calcite neutralisation etc. The electrical conductivity (EC) was observed to be very high (> 10 mS/cm). Sulphate concentrations were 11890 mg/L. Major cations in AMD included Na, Ca, Mg,

Al, Mn and Fe. The dissolution of silicate minerals such as feldspar, kaolinite and chlorite can account for the presence of some of these elements in AMD.

Figures 1 and 2 shows the pH and EC trends for AMD neutralization with fly ash when mixed in different liquid to solid ratios. The pH increases as the AMD to fly ash ratio decreases.

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Cg ] 14 ERE ar ZETR Arr TERA [ERE

Tn dl] raion eT R ee en |e Ratio 1.501 - 4 yr 4 ) : , IZ 0 I 0 5 15 40 60 120 180 240 360

Time (mins) . Figure 1: pH trends of AMD neutralization with fly ash showing the effect of different AMD to fly ash ratios.

Using an AMD to fly ash ratio of 03:01 in the neutralization reaction of this highly contaminated AMD, a pH of 6.33 could be achieved, whereas by using more fly ash (AMD to fly ash ratio of 1.5:01), the reactions could achieve alkaline pH, reaching a pH as high as 12. A buffer region between pH 4 to 6.5 was observed for all the ratios for a considerable period during which time most metals precipitate out of the solution.

The EC trends (Figure 2) showed a gradual decrease in the EC values observed over reaction time. It was also noticed that the lower the AMD to fly ash ratio the better the efficiency in reducing the EC.

Table 2 summarizes the major and trace element removal efficiency as a function of pH for fly ash and AMD neutralization reactions.

DrG Ref: 666614 gm Sanmae nea a ee : T ai —— Ratio 2.51

SC EEE TREAT | L Ratiool

S 4 Line, EE BLN % —s— Ratio L501 2 oi : . 0 ' ! 0 5 15 40 60 120 180 240 360

Time(mins)

Figure 2: EC trends of AMD neutralization with fly ash using different

AMD to fly ash ratios.

Table 2: Major and Trace element removal as a function of pH for fly ash and : 5 AMD neutralization : Ratios Final Mn Fe Al Mg Zn Ni Cu Pb S00.

AMD: fly pH Final Final Final Final Final Final Final Final Final ash Conc. Conc Conc Conc Conc Conc Conc Conc Conc mg/I mg/l mg/l mg/l ma/| mg/l mg/l mg/| mg/l

AMD 2.69 226.3 5599.9 1068.1 2661.7 49 6.95 0.355 0.314 11888.1 3:1 6.33 56.65 293.3 2.85 636.9 1.30 0.58 0.045 0.019 5483.3 2.5:1 8.72 5.15 52.3 3.26 618.1 1.20 0.134 0.055 0.041 2414.3 2:1 9.47 1.11 43.2 2.35 200 1.26 0.088 0.073 0.03 2508.1 1.5:1 12.05 0.13 4.7 9.41 1.5 0.737 0.051 0.034 0.015 4570.7

Table 2 shows that the maximum sulphate removal from this highly contaminated AMD was achieved in the case of the 2.5:1 ratio followed by the 2:1, 1.5:1 and 3:1 ratios. Although no exact trend was followed, the lower AMD to fly ash ratios could achieve higher sulphate removal (except 1.5:1) indicating 10 that the higher quantities of fly ash present in the reaction mixture lead to the dissolution of more CaO and subsequent formation of Gypsum as determined by

XRD, which accounts for the sulphate removal. These reactions exhibited a large buffering capacity at pH 5.5-6.5 which resulted in a longer contact time and subsequent higher sulphate % removal by adsorbing on to Fe hydroxides and Al, Fe oxyhydroxysulphates.

All the ratios achieved good metal attenuation as can be observed from Table 2.

Although the 3:1 ratio could achieve only a pH of 6.33 in the time of reaction,

Fe, Zn, Ni, Cu and Pb were removed quite efficiently in this reaction. Thus the

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: pH range achieved by this reaction covers the optimum pH range required for these elements to precipitate. Cu (for 2:1 ratio) and Pb (for 2.5: 1 ratio) showed

N some variability in removal as the pH increased. This can be attributed to the formation of hydroxo complexes for both the elements at the pH range 8.7-9.5. ; 5 Mn concentrations decreased with an increase in fly ash in the reaction, with 99.9% removal of Mn being obtained at a pH of 12.05 (1.5:1 ratio). Similarly

Mg shows efficient removal at the highest pH. This corresponds to the optimum precipitation pH of Mg as Mg(OH),.

Solid residues as Backfill Material

Referring to Figure 9, a first application for a fly ash derivative being a process for producing a backfill for a mine in accordance with the invention, generally indicated by reference numeral 10, is shown. :

In a first neutralisation step (16), the acid mine drainage (14) and fly ash (12) are mixed at different ratios ranging from 1:1 to 10:1 depending on the neutralization capacity of the fly ash (12) and the degree of contamination of the AMD (14). Preferably the ratios are 3:1 to 7:1 for highly contaminated AMD containing significant levels of sulphate Fe and Al. When the neutralization reaction between the fly ash (12) and the acid mine drainage (14) reaches an alkaline pH, i.e greater than pH 7, the reaction can be stopped. However, the reaction can be allowed to continue until a pH 9 or greater is obtained to ensure that most of the undesirable metals are removed from the resulting process water.

The reaction solution is allowed to settle for approximately one hour, which is required for the liquid and solid phases to separate (18). It is noteworthy that the settling time of the solids derived from treatment of the acid mine drainage with fly ash is far less than the settling time of solids derived from limestone treatment of the acid mine drainage and the sludge density is much higher.

The treated process water (20) is recovered and the partially dewatered solid residue (22) is pumped out of the reaction tank. The partially dewatered solid residue (22) thus recovered is transported as a slurry (24) by pipeworks to a

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} filling site at a mine or, if fully dewatered, is crushed and mixed thoroughly with water to attain homogeneity before being transported as a slurry (24) to a filling : site at a mine by pipe. If required, the solid residue (22) can be mixed with water to form the slurry (24) with optimum density so that the solids do not . 5 cause any pipe blockages during transportation. The water used to prepare the slurry is either process water (20) or tap water. The solid residues can also be brought to an appropriate viscosity for slurry or paste transportation. Finally, the solid residues are placed underground. Optionally cement or fly ash can be added to the solid residues (22) to induce more cementing properties. The 10 underground placed solid residues harden over time, which prevents the ingress of air and thereby reduces the risk of further acid mine drainage being generated.

In order to ascertain the suitability of solid residues for backfilling, flowability experiments were carried out on the solid residues obtained from the reaction between fly ash and AMD. The flowablity tests were performed by means of a marsh cone. This was done by adding tap water to solid residues containing 3% binder (ordinary Portland cement) until the time for 1 L to run out had decreased to less than 10 seconds. The resulting value was taken as the required water content for make-up of pumpable slurry. The waste material from the reaction between fly ash and AMD had a solids fraction of about 0.7. It was necessary to add water in order to achieve a reasonable flow rate in separated, dewatered solid residues. Ordinary tap water was used for this purpose. Table 3 gives parameters of the flow test using the marsh cone for solid residues. The addition of 605.2 g of tap water allowed the slurry to flow through the cone in 9.8 s which is the minimum time in the experiments conducted.

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: Table 3: Physical parameters of slurry flow out for solid residues. } Water Total Solids RelativeRun outFlow rate Water to Flowed Flowed mass water (g) density time (s) (Tot g/s) solid ratio water (L) solid (g) mass (g) volume (L) 382.2 382.2 733 1.658 - - 0.521 0.443 0.557 ; 136.6 518.8 669 1.568 - - 0.776 0.519 0.481 38.7 557.5 653 1.547 20.0 60.53 0.854 0.537 0.463 18.8 576.3 645 1.537 13.5 90.47 0.894 0.545 0.455 28.9 605.2 634 1.523 9.84 12593 0.954 0.557 0.443

The run out time of slurries was found to increase with an increase in relative density (RD) (Figure 3). Accordingly, an increase in RD was responsible for a decrease in flow rate of the slurry (Figure 3). This is a consequence of the successive additions of water during the marsh cone test process.

B IEE eee ae 0 0 I EE NN 0 i I I ES 5 1.547 1.537 1.523

Slurry RD ’

Figure 3: Marsh cone run out time and flow rate as a function of slurry relative density.

In another study the flowability of solid residues was shown to be a function of fly ash particle size, when using fly ash of different particle size from the same power station. The water that was used for these experiments was the post- neutralization water that was recovered after the reaction between fly ash and

AMD. These results show the need to add water for ensuring correct relative density for run out and solids flow rate in the Marsh cone tests and indicates that solid residues need not be completely dewatered prior to their work-up into slurries for use as backfill materials, which will reduce the operational costs of backfilling.

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Laboratory tests done on different columns containing solid residues alone and solid residues mixed with different proportions of fly ash or cement indicate that

A unblended solid residues have a significant and extensive capability to passively treat acid mine drainage over time.

Two replicated columns (height 300 mm; diameter 105 mm) were packed with solid residues, each column containing 2 kg of solids. Similar replicated columns were set up for the solid residues, and solid residues blended with 5%, 25% and 40% unreacted ash as well as solid residues blended with 6% ordinary Portland cement (OPC) for comparison. In order to exclude variability in the composition of AMD, Simulated AMD (SAMD) was prepared and percolated through the columns. It was formulated using soluble salts of the major elements typically found in AMD (Fe, Al, Mn and SO,). The salts were dissolved in 0.005 M H,S0, solution to prevent immediate precipitation of ferric iron. Metal salts used were anhydrous ferric sulphate, hydrated ferrous sulphate, hydrated aluminium sulphate and manganese (II) nitrate tetrahydrate. Various 350 ml aliquots of the SAMD were percolated through the packed columns over a period of 165 days. The permeate was collected at the bottom of the columns after each aliquot had passed through the respective columns for analysis.

The initial pH of the SAMD that was percolated through the columns was 1.83 and the principle composition of SAMD was as follows: Al (908.67 mg/l), Mn (200.13 mg/l), Fe-total (4726.36 mg/l) and SO, (14407 mg/l). In the case of unblended solid residues permeate, the pH of the permeate remained alkaline (pH=8.2) till 81 days and then decreased to acidic pH. For solid residues blended with 5%, 25% and 40% fly ash permeates, the pH remained alkaline for 110, 97, 81 days respectively. For solid residues blended with 6% OPC permeate, the permeate pH became acidic within 22 days. The efficiency of solid residues by itself or blended with different % of fly ash or OPC as a passive treatment option was assessed by calculating the total amount of elements (mmol/l) in SAMD before and after (permeate) percolating through the columns.

Table 4 details the efficiency of each combination in terms of % removal of each element.

DrG Ref: 666614 i Table 4: Table showing efficiency of different columns in terms of % removal of . each element

Column A Fe Mn sO, (%removed) (%removed) (%removed) (%removed) ] SR : 99.57 90.32 73.79 83.29

SR+5% fly ash 99.86 96.63 84.61 82.33

SR+25% fly ash 99.82 95.34 82.83 89.07

SR+40% fly ash 99.70 95.32 82.55 90.32

SR+6% OPC 96.00 94.86 91.31 86.94

In general, it can be seen that solid residues columns blended with fly ash and

OPC have a higher capacity than the solid residues column itself to passively remove SO, (except solid residues+5% fly ash), Fe and Mn. This is a result of alkalinity from CaO dissolution in each column raising the pH of the SAMD solution that results in the precipitation of those elements from solution.

Moreover dissolution of CaO from the added fly ash and formation of Ca (OH), from OPC hydration in the presence of SO, results in precipitation of gypsum.

The solid residue was competent in the removal of Al. This waste residue was observed to have a high capacity to absorb Al at low pH. The precipitated compounds are, however stable in all the columns and do not re-enter the leachate solution. The concentration of these elements only started to increase in the leachate once the alkalinity in each column had been exhausted and the pH decreased to 4.

A study was conducted to establish the development of strength of the solid residues when used as backfill. The solid residues in addition to treating AMD passively, should also provide mechanical strength to support the overburden, moreover the unconfined compressive strength (UCS) are essential for the rock mechanics to model the force interaction and lateral load bearing of cemented - strength.

For the strength testing, the wet solid residues were tested on their own or blended with a pozzolanic binder (Castle cement) at rates of 1%, 3% and 6% on a dry weight basis. The unblended or biended wet solids were slurried with the required amount of tap water as determined from the marsh cone test for

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. flowability. Then the slurries were poured into cylindrical curing tubes and placed in plastic bags, a bottle of water being placed next to them to maintain a the required level of humidity for the curing process. Unconfined compressive strength testing was performed according to ASTM, 1993, for the cured } 5 cylinders.

Strength measurement of the solid residues blended with varying rates of OPC (1%, 3%, 6%) on a dry weight basis was done as a function of time. Strength development of solid residues over time was found to be proportional to the % of binder used (Figure 4). The sample with 6% binder gained considerable strength over time and reached a peak stress of about 2 Mpa indicating the suitability of solid residues as a backfill material. = TT 1 - 6% Binder = 0.5 i , ] 29 91 187

Curing Period (Days)

Figure 4: Unconfined compressive strength of solid residues formulated with tap water

The samples gained considerable strength within the first 28 days of curing indicating the suitability of process water for the slurry making. This early accelerated strength development was attributed to the formation of gypsum on hydration of the cement binder, with the process waters acting as a rich source of sulphates.

Zeolite Synthesis from the solid residues and decontamination of post neutralization waters

For the synthesis of Zeolite, fly ash and AMD were obtained and neutralized according to the specified method. After the neutralization reaction, the solid

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} residues were collected, dried for 12h at 100°C and crushed. Prior to zeolite processing, the major oxides in the solid residues were determined with XRF

A analysis. The [Si]/[Al] ratio was determined to be 2 for the solid residues used in the high temperature hydrothermal method of synthesis and 1.6 for the low ; 5 temperature method, both of which proved to be a sufficient quantity of silica compared to aluminium, required for the successful synthesis of zeolites. The [Si]/[Al] ratios were different because the composition of the starting solid residues were slightly different because of different levels of contaminants in the

AMD used. The procedure used for the high temperature synthesis method was a alkaline hydrothermal method. The dried solid residues were fused with a

NaOH solution at 600°C for 2 h. The fused product was aged for 8h in deionised water and crystallized at 100°C for 24 h. Finally, the collected product was rinsed with deionised water and dried overnight at 70°C. The procedure used for the low temperature synthesis, consisted of an ageing step in which the solid residues were mixed with a highly concentrated (2M) NaOH solution. Then the slurry underwent a thermal treatment at 100°C under a reflux system for 4 days. The synthesised product was then washed and dried overnight at 100°C.

The zeolite containing solids produced by both methods were characterized with

XRD and BET-N, methods.

Figure S shows the XRD spectrum for the zeolite that was synthesized at high temperature (600°C). The peaks visible on the spectrum indicate the presence of Zeolite Y or Faujasite phase. As per the XRD spectrum, it can be also seen that Zeolite Y is mixed with Sodalite which could be due to an excess of NaOH in the fusion treatment. This mixed phase results in some reduction of the surface area of the Zeolite Y. The surface area determined by BET-N, analysis was 327 m?/g. Figure 6 presents the XRD spectrum of the Zeolite that was synthesized . at lower temperature (100°C). The crystallography indicates that the product was a combination of zeolite P and mullite.

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- 5 1000 oo TER | 2 tg gh gt SEER || 8200 TAR ¥ 1 LO AER 4 500 100 TS Wl BA ; 0 0 15 25 35.045 55 6S 5 15 25 35.045 55 65

Figure 5: XRD spectrum of the Figure 6: XRD spectrum of the product synthesized at 600°C product synthesized at 100°C

The mullite phase represents unconverted fly ash and its presence reduced the

S total surface area of the synthesized product, which was 67 m?/g. The type of zeolite formed is affected to a great degree by the ratio of Si to Al in the starting material. Zeolite P requires a ratio of 1.2 whereas faujasite requires a ratio of approximately 2. At low temperature synthesis the Si source in the starting material may not have sufficiently demineralised and thus resulted in the dense mullite phase. The synthesized zeolites were subjected to various leaching protocols and it was observed that no significant leaching of toxic elements took place. The leachate water was of high quality meeting with the standards of quality for irrigation waters.

Zeolites were contacted with the post-neutralization water at a concentration of 10 g/L in order to remove any remaining trace elements. The post-neutralization water was separated from the solid residues and a sample was filtered with a 0.45 ym membrane filter and analysed with ICP-MS. The removal capacity of the zeolite synthesized was compared with the capacity of a commercial faujasite, CBV 400. The mixture was stirred for 1 h with an overhead stirrer.

The initial and final pH of the water was recorded. At the end of the experiment, the water was filtered with a 0.45 um membrane filter and analysed.

The post-neutralization waters were decontaminated using the Zeolite obtained from the low temperature synthesis i.e. Zeolite P with some mullite phase in it.

Table 5 shows the removal efficiency of primary treatment (neutralization of

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. AMD using fly ash) to remove various elements from AMD and the efficiency of

Zeolite P to remove various elements in the post-neutralization waters. It also

A shows the toxic element removal efficiency of commercial zeolite in the post- neutralization waters. The primary treatment of AMD with fly ash achieved a . 5 significant removal of various toxic elements from the solution (Table 5), such as Fe, As and Cd which were greatly removed. However, the treatment left concentrations of Na and Ca unchanged and resulted in the signature release of

Sr, Ba and Mo from fly ash into the solution. During the secondary treatment, the removal percentage of Sr and Ba were 82.8% and 95.9% respectively.

Using commercial Zeolite Y the removal percentage of Sr, Ba was very poor with the later being leached out into the solution. Fe and Ca concentrations were significantly removed by Zeolite P where as there was a poor performance by the commercial zeolite to remove the same elements. However, Mo and As was efficiently removed by Zeolite Y (CBV 400) when compared to Zeolite P. The removal capacity of Cd was more or less similar for both the zeolites. Na was ion-exchanged out of both zeolites by the preferential uptake of the toxic elements.

Table 5: Composition of AMD and treated waters after contact with ash and a zeolite P containing solid prepared from solid residues

Primary treatment Secondary treatment with with Commercial

Elements AMD fly ash Synthesized zeolite Y zeolite P CBVv400

Fe (mg/L) | 7841.0 1.5

Ca (mg/L 340.5 383.0 262.8 313.0 . _Na (mg/L 305.0 208.7

Sr (mg/L 14.4

As (pg/L 0.5

Cd (pg/L 0.4

Ba (u g/L 85.1

Mo ( ub g/L 325.5 295.4 231.4 pH 4.9

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. Ashwalling Studies . In a further example of how to passively treat AMD, with fly ash, fly ash was ) used to pack 100 mm diameter, perspex columns of different sizes: 1.5, 1.0, 0.5 and 0.25 m in length. Each column type was duplicated and given the annotation A or B. The mass of each duplicate column before and after filling was noted to ensure that they contained the same amount of fly ash. Raw AMD was percolated through the columns by gravity feed for 45 days or until the alkaline permeate from the columns returned to pH 8.5. Once this end point was achieved, the raw feed of AMD flow was stopped and the ash removed from the perspex as a solid column. Samples of ash were collected at varying depths in the columns being the top, middle and bottom parts of the columns. A sample of the original fly ash, not submitted to percolation of AMD, was also collected for analysis. The collected ash samples were analysed using XRF,

XRD, FTIR, SEM-EDS and Raman Spectroscopy. The raw AMD before the percolation and the leachate/permeate that was collected at the bottom of the columns were analysed for pH, EC and different elemental concentrations by using ICP-MS and IC.

Figures 7 and 8 show the permeate trends of pH and EC for the period of 45 days. The pH of the permeate of all the columns increased immediately after the percolation of AMD from 2.88 to above 12. The pH remained above 11.5 for the period of the test for most of the columns. The EC levels largely decreased and remained low all through the experiment except in the shortest columns. )

RL / fe Sim = SSS S—— gE 0 5 10 15 Days 5 x 35 40 45

Figure 7: pH trends for different lengths of fly ash columns

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. 14 A. 12 PAN

IRV="Y SN SN Ey

B= . 4 Be Daren del 0 = 0 S 10 15 pays 5 30 35 40 45 :

Figure 8: EC trends for different lengths of fly ash columns

The permeates were analyzed for sulphates and other toxic metal concentrations. These values were compared with the original concentrations in the raw AMD and a 99% reduction of sulphate concentrations was observed for all the columns. Elements Al, Ba, B, Cr, Cu, Fe, Mn and Zn were analyzed for their concentrations in the raw AMD feed and after passing through the fly ash columns. The concentration of all the major minor and trace elements was greatly reduced except for Ba, and B. These two elements had leached into the solution from the ash during the passive treatment of raw AMD.

In order to understand the mineralogical changes in the ash that was exposed to AMD, the ash columns were divided into three sections (Bottom, Middle and

Top) and were characterized using different techniques. Table 6 shows the results for the XRF analysis of the columns. It was apparent from the XRF results that Si and Al were more mobile and concentrated toward the bottom parts of the fly ash column whereas Fe and sulphate was immediately precipitated from raw AMD upon contact with fly ash and was dominant in the top sections of the column. The composition of lower sections of the longer ash columns remained largely unchanged. The CaO content was generally not noticeably depleted by the percolation of the raw AMD through the ash columns.

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: Table 6: XRF for fly ash columns. ] Mass % SiO, Al,O; SO; CaO Fe,03 TiO, P,0s MgO Na,0 K,O LOI Total - ) Fly Ash 0.25 A Top 36.9 29.8 14.6 9.98 3.69 1.32 1.28 0.67 0.89 0.84 10.4 110 ] 0.25 B Top 36.1 29.6 15.4 9.97 4.01 1.38 1.34 0.73 0.82 0.64 10.6 111 0.25 A Middle 38.2 26.0 9.83 8.88 10.1 1.27 1.69 2.04 1.14 0.89 5.20 105 0.25 B Middle 38.3 26.2 9.80 8.85 9.52 1.32 1.46 2.65 1.15 0.79 6.00 106 0.25 A Bottom 42.0 28.5 5.80 12.3 3.10 1.84 1.37 3.20 0.91 1.05 4.80 105 0.25 B Bottom 41.8 28.1 7.08 9.59 6.53 1.50 1.30 2.24 1.02 0.83 6.00 106 0.5 A Top 36.8 26.0 11.0 9.25 9.82 1.45 1.29 2.58 1.09 0.77 6.40 106 0.5 B Top 36.3 25.6 10.4 9.30 11.4 1.49 1.18 2.47 1.03 0.81 6.50 106 0.5 A Middle 44.7 30.4 3.37 11.3 2.38 1.83 1.57 2.57 1.09 0.90 3.60 104 0.5 B Middle 40.9 27.7 8.16 10.9 3.66 1.44 1.41 4.00 1.12 0.78 5.40 105 0.5 A Bottom 47.3 31.6 1.20 9.74 2.31 1.51 1.86 2.33 1.17 0.96 3.40 103 0.5 B Bottom 47.3 31.6 1.23 9.98 2.42 1.77 1.38 2.23 1.02 1.07 3.50 103 1.0 ATop 36.8 25.9 10.7 10.1 9.14 1.51 1.41 2.66 0.94 0.79 6.00 106 1.0 B Top 37.8 26.4 10.5 9.72 7.49 1.49 1.57 3.33 0.86 0.78 6.00 106 1.0 A Middle 47.2 31.5 1.25 10.2 2.25 1.53 1.72 2.22 1.16 0.99 3.80 104 1.0 B Middle 47.1 31.5 1.20 10.1 2.34 1.66 1.96 2,22 1.07 0.90 3.60 104 1.0 A Bottom 45.4 31.0 1.40 11.4 2.71 1.79 1.84 2.43 1.03 0.95 3.60 104 1.0 B Bottom 47.3 31.2 1.15 10.2 2.57 1.69 1.72 2.16 1.17 0.93 3.60 104 1.5 A Top 38.7 26.9 9.06 10.7 6.73 1.48 1.33 3.25 0.98 0.86 6.20 106 1.5B Top 37.8 26.3 10.3 9.20 8.16 1.32 1.78 3.22 1.09 0.79 6.40 106 1.5 A Middle 46.2 31.6 1.46 10.4 2.46 1.74 1.72 2.35 1.08 0.98 3.60 104 1.5 B Middle 46.8 31.1 1.75 10.2 2.66 1.68 1.48 2.21 1.17 0.99 3.70 104 1.5 A Bottom 45.5 31.2 1.30 11.4 2.99 1.89 1.60 2.03 1.07 1.03 4.40 104 1.5 B Bottom 47.2 31.5 1.05 10.3 2.31 1.91 1.64 1.99 1.05 1.01 4.30 104

Based on the SEM, XRD and Raman results, it was noticed that upon the passage of AMD, mineral phases such as gypsum and calcite were precipitated in the fly ash column. Although no major mineralogical changes were observed using the characterisation techniques, the fact that the columns blocked over time indicated agglomeration or coalescence of ash particles by insoluble precipitates that formed upon contact of AMD with the fly ash. This showed the potential of ash walling as a reactive barrier to direct AMD flows or passively treat AMD flows.

Several mechanisms are involved in treating AMD with fly ash for the removal of sulphate and other contaminants: 1. Initial dissolution of CaO from fly ash in the

DrG Ref: 666614

. presence of acidic AMD, Ca** accumulates in reaction mixture, interacts with

SO4* to form gypsum, dissolution of Ba and Sr salts from fly ash and ’ consequent interaction with SO,* also leads to formation of Barite (BaSO,) and

Celestite (SrS0,) or a combination of the two salts. 2. As the pH increases >3, ; 5 Fe?" precipitates to form amorphous ferric hydroxides and oxy hydroxides which are known to have large surface areas and adsorb some of the SO,%. In the presence of oxygen oxidation of Fe?* occurs and is maximum at pH 5-7, hydrolysis of the resulting Fe** results in the formation of more amorphous ferric hydroxides which adsorb more of the SO,%. At this pH range a significant decrease in SO4* concentration is noted. Formation of amorphous Al (OH); and incorporation of SO, at pH>4 is also believed to contribute to reduction in SO content. 3. At pH > 9 a mineral phase known as ettringite (Cag Al; (SO4*)3(OH)) forms and again contributes to decrease in S0,% levels, but this mineral phase is only stable at pH >9.

DrG Ref: 666614

Claims (20)

1. LE cLAIMs CL . 1. A method of using fly ash including the steps of mixing acid mine drainage i with fly ash; and of allowing the acid mine drainage to react with the fly ash in a neutralisation reaction, to form a solid residue product and pH neutral process water.
2. 2. The method of using fly as claimed in claim 1, in which the fly ash consists of at least 70% of the elements SiO,, Al,O; and Fe,0s.
3. 3. The method of using fly ash as claimed in claim 1 or 2, in which the acid mine drainage and fly ash are mixed in a ratio of between 1:1 to 40:1.
4. 4. The method of using fly ash as claimed in claim 3, in which the acid mine drainage and fly ash are mixed in a ratio of between 1:1 to 10:1.
5. 5. A method as claimed in any one of the preceding claims, in which a zeolite made from fly ash is contacted with the process water recovered after the neutralisation reaction to remove toxic elements from the process water.
6. 6. A method as claimed in any one of claims 1 to 4, in which a zeolite made from the solid residue product is contacted with the process water recovered after the neutralisation reaction to remove toxic elements from the process water.
7. 7. A method as claimed in claim 5 or 6, in which the toxic elements include at least one of Fe, Ca, Na, Sr, As, Cd, Ba, Mo.
8. 8. A method of using fly ash including the steps of mixing acid mine drainage with fly ash in a neutralisation reaction; of extracting a solid residue from the neutralisation reaction; and of using the solid residue in a mine for backfilling operations.
9. 9. A method as claimed in claim 8, in which the acid mine drainage and fly ash are mixed at ratios between 1:1 to 10:1 in the neutralisation reaction. DrG Ref: 666614 va

   .
10. 10. A method as claimed in claim 8, in which the acid mine drainage and fly ash are mixed at ratios between 4:1 to 7:1 in the neutralisation reaction. )
11. 11. A method as claimed in any one of claims 8 to 10, in which the neutralisation reaction is stopped after a pH greater than pH 7 is obtained.
12. 12. A method as claimed in any one of claims 8 to 11, in which the neutralisation reaction is stopped after a pH greater than pH 9 is obtained.
13. 13. A method as claimed in any one of claims 8 to 12, in which the neutralisation reaction solution is allowed to settle for at least one hour to allow separation of the solid residue from the neutralised process water.
14. 14. A method as claimed in any one of claims 8 to 13, in which the solid residue is converted into a slurry for transportation to a mine.
15. 15. A method as claimed in claim 14, in which the slurry is formed by mixing the solid residue with either neutralised process water or tap water or by partial dewatering of the resultant sludge from the neutralisation reaction.
16. 16. A backfill for a mine including a solid residue product of a neutralisation reaction between acid mine drainage and fly ash.
17. 17. A backfill as claimed in claim 16, in which the solid residue is in the form of a slurry.
18. 18. A backfill as claimed 16 or 17, which includes fly ash and a cement additive added to the solid residue.
19. 19. A method of using fly ash including the steps of placing the fly ash in a mine void so that it can be contacted and react with acid mine drainage seeping though the mine void.
20. 20. A method of using fly ash substantially as hereinbefore described. DrG Ref: 666614

    ” N

    . 21. A backfill for a mine substantially as hereinbefore described. . Date: 1 February 2008 . John Spicer S DR GERNTHQLTZ INC Patent Attorneys of Applicant(s) P O Box 8; Cape Town 8000; South Africa 30 Union Road; Milnerton 7441; South Africa Tel: (021) 551 2650 Fax:'(021) 551 2960 DrG Ref.: 666614 t:\files\14\666614\6666 1 4spec.doc DrG Ref: 666614