Fly ash zeolite synthesis for borehole water softening
Lovejoy Nyoni1, Vusizwe Nyathi2, H Madhlunga3, Joel Tshuma4*,
1,2,4*
- Department of Chemical Engineering, National University Of Science and Technology, P
O Box AC 939, Ascot, Bulawayo, Zimbabwe
Telephone: 002639282842 or 00263773405259
Fax: 00263286803
* Email: joel.tshuma@nust.ac.zw or jotshuma@gmail.com
*Corresponding author
3
Zimbabwe Power Company, Bulawayo Power Station, P.O. Box 1803, Lobengula St. /Khami
Rd, Bulawayo, Zimabwe
Telephone: 09-78228
Fax: 78240
ABSTRACT:
In this research a study was carried out to investigate the possible use of fly ash to produce a
zeolite that can be used for softening borehole water. Fly ash samples used were obtained from
Harare power station. Preparation of the sample involved ash acid activation using HCl followed
by alkali fusion using NaOH at 600ºC.[10,14] The sample which was then treated hydrothermally
using distilled water, was filtered off.[14] The residue was then dried in an oven to produce the
zeolite sample which was used to soften samples of borehole water with a total hardness of
770ppm. The parameters which were under investigation were the NaOH amount added,
residence time and the zeolite sample mass used. Total hardness and calcium hardness
percentage removal efficiencies of 67% and 83% were achieved respectively. The conclusion
drawn from the study is that fly ash can be successfully beneficiated to produce a zeolite that can
be used to economically and efficiently used to soften hard borehole water.
Keywords: Fly ash, hardness, softening, removal efficiency.
1. INTRODUCTION
The Zimbabwean Thermal Power Stations use coal as a source of fuel. In the combustion of this
coal large quantities of fly ash are produced. For example the Harare Power Station is estimated
to be producing 28 tonnes of fly ash over a period of 24hrs when operating at full plant capacity
that is at 30MW load. At this production rate the landfills where the fly ash is disposed cannot
accommodate such large volumes. Hence the fly ash is accumulated within the plant. The
accumulation of fly ash in land fills or land surfaces which are not lined can result in the
contamination of underground waters [1,2]. The fly ash exposed to the surface when dry can
generate dust which is a hazard to both the environment and to health [ 3,4 ] Strigent
environmental regulations sanction thermal power stations from accumulating large amounts of
fly ash. The Environmental Management Agency in Zimbabwe is placing severe restrictions
regarding the disposal of fly ash [5]. Hence the Zimbabwe Power Company is having serious
problems in the disposal of fly ash. Therefore, this increase in the requirements in the
environmental protection field have induced search for more effective, economic and
ecologically safe solutions to deal with pollution and waste not only in Zimbabwe but in the
whole world[6] Hence the validation and implementation of efficient, simple mitigation measures
is indicated for compliance and environmental protection.
These thermal power stations are not only facing the problem of disposing fly ash but they are
also encountering the problem of inadequate water supplies from Water Authorities. This has led
to some of the coal fired power stations resorting to the use of borehole water to supplement the
water supplies. This borehole water has high quantities of calcium and magnesium ions as is
typical of any underground water supply.[ 7,8 ] The presence of these ions in the boiler water
system will cause scaling in the boiler tubes and thus reduce thermal efficiency of the boiler.[ 9 ]
Therefore the water that is fed into the boilers should be pre-treated to reduce its hardness to zero
[10]. There are many technologies in existence for the removal of these hardness ions. These
include soda lime softening, dealkalization, reverse osmosis, electro dialysis, continuous
deionization, activated carbon etc [
]. These technologies are however very expensive.
Therefore there is a need to develop an economic, effective and environmentally sustainable
technology to soften borehole borehole water to be used by Zimbabwean Thermal Power
Stations.
In regard to the two problems encountered by the Zimbabwe Thermal Power Stations, the aim of
this project is to synthsise zeolite from the fly ash produced by these power stations and use it to
soften the borehole water. Therefore this project will render a double solution to the problems
encountered by the power station by removing hardness of water fed into the boilers and that of
fly ash disposal since this fly ash will be used in synthesizing the zeolite.
This project is therefore going to render a double solution encountered by the power station by
removing hardness of water fed into the boilers and that of disposal since this fly ash will be used
in synthesizing the zeolite.
Theoretical Background
The term Zeolite is derived from the Greek verb Zein which means to boil and the Greek noun
lithos which means stone. Therefore literally the name Zeolite means boilng stone. This name
was created by the Swedish Mineralogist Axel Cronstedt in 1758 when he noticed that a Zeolite
specimen would swell and then crackle and pop when heated [14 ]. The zeolites are framework
silicates consisting of interlocking tetrahedrons of SiO4 and AlO4. In Zeolites the ratio (Al + Si
)/O is equal to ½. [15] The alumino-silicate structure is negatively charged and attracts the
positive cations that reside within. The Zeolites have large vacant spaces or cages in their
structures that allow space for large cations such as sodium, potassium, barium and calcium and
even relatively large molecules and cation groups such as water, ammonia, carbonate ions and
nitrate ions. In the more useful zeolites, the spaces are interconnected and form long wide
channels of varying sizes depending on the mineral. These channels allow the easy movement of
the resident ions and molecules into and out of the structure. Zeolites are characterized by their
ability to lose and absorb water without damage to their crystal structures. The large channels
explain the consistent low specific gravity of these minerals [16]. These Zeolites can occur
naturally [17] or can be synthesized[18]. Natural zeolites have enough impurities to make them
less useful for some specialized industrial applications, so synthetic zeolites are designed for
those specific applications [19]. In this project fly ash has been selected for the synthesis of a
Zeolite because it contains high quantities of silica and alumina which are the main constituents
of any naturally occuring Zeolite [20].
The silicon dioxide is present in two forms that is amorphous and crystalline. The amorphous
silicon dioxide is mostly smooth and rounded whilst the crystalline form is sharp and pointed and
is in the form of quartz and mullite.[11]
The fly ash zeolite to be produced in this project on the other hand is a crystalline, hydrated
alumina-silicate of alkali and alkaline earth cations, having a three dimensional silicate
structure.[11] The zeolite made in this research study was made from fly ash produced from the
combustion of pulverised nut peas duff coal supplied from Hwange Colliery Company. The
zeolites have some common properties such as porosity, size-selective inclusion and sieving,
reversible hydration and ion exchanging ability. Within the pores, the alkali metal cations and the
water molecules are free to move around.[11,12,13] It is this feature that allows zeolites to exchange
charge compensating cations with ease.
The synthesized fly ash Zeolites will then be used for softening borehole water (to be used in
boilers) by removing Ca2+ and Mg2+ ions.
3.0 MATERIALS AND METHOD
3.1: Equipment

XRF

Nabertherm furnace HTC 03

Lab oven

Hotplate

Ph meter

Mass Balance

Sieves

Beakers (250ml, 500ml)

Conical flasks (250ml, 500ml and 1litre)

Measuring cylinders

Pipette (5ml, 10ml)

Biuret (0-100ml)

Spatulas

Conical flask stoppers

Thermometer (0-110)

Stirrer/ mixing rod
3.2: Reagents:

Hydrochloric Acid

Sodium Hydroxide

Ammonium buffer solution

Total and calcium hardness indicator reagents

Distilled water

Borehole water

Fly ash
3.3: Reagents Preparation
3.3.1 1M Hydrochloric Acid Preparation
3.3.2 1M Sodium Hydroxide Solution
3.4: Fly ash sampling
The coal fly ash samples used in this study were collected directly from the electrostatic
precipitator hoppers of Hwange thermal power station. A sample of 15kgs of fly ash was
sampled randomly from the electrostatic precipitators hoppers to ensure that a representative
sample was obtained .The fresh coal fly ash sample was put in a sealed plastic bag devoid of air
to avoid external contamination. The sealed plastic bag was stored in a dark, cool cupboard away
from any heat source, direct sunlight or fluctuating temperatures.
3.4.1 Preparation of Fly Ash Sample
The fly ash collected in 3.4 was sieved using a 75 microns sieve. The underflow fly ash sample
was heaped and mixed thoroughly. A cone and quarter method of sampling was applied to
collect 3 2g samples of a homogenous fly ash mixture which were taken for XRF Spectroscopy
Analysis.
3.4.2: Preparation of fly ash zeolite
i.
The fly ash sample from 3.4.1 was Calcined at 900°C for 2hours in a Nabertherm HTC
03 furnace.
ii.
The sample was removed from the furnace after 2hours and left to cool at room
temperature.
iii.
2.1kg of the calcined sample from ii was weighed and mixed with 2dm3 of 1M HCl.
iv.
The mixture was left to dry overnight.
v.
The prepared sample in (iv) was divided into 4 portions of 500g each and sealed in plastic
bags which were labelled A,B,C and D respectively.
vi.
100g of fly ash from each of the four plastic Bags were poured into 4 500 ml beakers
labelled A,B,C and D respectively
vii.
40ml, 80ml, 120 ml, 160 ml of 1.16 M [6] Sodium Hydroxide solution were poured into
the beakers labelled A, B, C and D respectively
viii.
The four beakers from vii were placed in the Nabertherm furnace HTC 03 and the
mixtures were fused at 600°C for 1hour [xxxx].
ix.
After 1 hour the samples were removed from the furnace and left to cool
x.
Each of the samples from (ix) were placed into respective 1litre conical flasks labelled A,
B, C and D.
xi.
The flasks were filled with distilled water to the three quarter full mark.
xii.
The slurries for each of the samples in (ix) were magnetically agitated the slurries and
treated hydrothermally for 1hour at 100°C.
xiii.
The samples from xii were placed into an oven at 100°C for 5hours. After 5 hrs these
samples were taken out from the oven and left to cool off at room temperature.
xiv.
The supernatant formed in the samples in (x) was decanted
xv.
The resultant precipitate was washed and filtered repeatedly with distilled water.
xvi.
The samples were dried at 80°C for 24hours in an oven.
xvii.
The dried synthesized zeolite samples from xvi were taken out of the oven and left to
cool.[14]
xviii.
The synthesized zeolite samples from xvii were sealed in air-tight plastic bags.
3.4:3 Procedure for softening borehole water
i.
1g of fly ash zeolite A was put into a 250ml beaker
ii.
100 ml of borehole water were poured into the sample from (i)
iii.
The mixture from ii was thoroughly agitated to obtain a homogenous mixture.
iv.
After agitation, the mixture was left to rest.
v.
10ml of the sample from the supernatant formed in iv were taken at 1 hour intervals for a
period of 8 hours
vi.
The samples obtained in v were analysed for total hardness and calcium hardness as
described in the Standard Methods [xx]
vii.
The above procedure from i-vi was repeated using 2, 4, 6,8 and 10 gram samples of
Zeolite A
viii.
The above procedure from i-vii was repeated using Zeolite B,C and D respectively.
4.0 RESULTS AND ANALYSIS:
4.1 Fly Ash Characterization
The XRF Spectroscopy results obtained on the analysis of the samples carried out in
Section 3.4.1 are shown in table1
Table 1. XRF Fly Ash Composition Analysis Results.
Parameter
Sample (% Weight)
1
2
3
SiO2
51.2
49.6
50.23
MgO
1.11
1.0
2.67
CaO
28.8
29.2
24.98
Al2O3
6.20
6.4
7.67
Na2O
0.78
1.2
0.78
K2O
1.92
1.89
1.93
Fe2O3
6.44
6.35
6.42
LOI
4.5
4.54
4.3
Average
(% Weight)
49.67
1.06
29.0
7.61
0.80
1.90
6.42
4.45
The results in Table 1 show the various constituents of the fly ash determined using the
XRF Spectroscopy Analysis. The most abundant compound in the fly ash analysed was
found to be Silica with an average % weight of 49.67. Among the remaining components
the Al2O3, K2O and Fe2O3 were found to be in quantities of 7.61, 1.90 and 6.42
respectively. This composition of basic oxides is consistent with sub-bituminous coal fly
ash. This fly ash can be classified as Class C because from Table 1 the total of the %
average weights of SiO2, Al2O3 and Fe2O3 is 63.70 which is above the minimum limit of
50% [7]. The high CaO value of 29.0 shown in the results confirms that the fly ash
analysed belongs to Class C. The loss on ignition (LOI) was found to be 4.45% implying
that the remaining weight of the ignited coal was very low.
The ratio [SiO2]/[Al2O3] is 7 indicating that it is well above 1.5 [x]. The CaO, MgO and
Na2O were found to have a weight percentage of 29.0, 1.06 and 0.80 respectively.
Therefore this chemical composition favours the formation of a faujasite type of Zeolite
[x1]. In conclusion the fly ash sampled and analysed is an appropriate raw material for
the alkaline hydrothermal conversion into Zeolite because it contains high quantities of
silica and alumina which are the main constituents of any naturally occuring Zeolite.
In table 1 the loss on ignition (LOI) is 4.45% implying that the total weight lost during
fusion was negligible
a) EFFECT OF NaOH DOSAGE
TOTAL HARDNESS REDUCTION
70,00
69,00
68,00
TOTAL HARDNESS (%) REDUCTION
67,00
66,00
65,00
64,00
63,00
62,00
61,00
60,00
40mg
80mg
120mg
160mg
63,90
64,68
65,32
65,45
CALCIUM HARDNESS REDUCTION
90,00
89,00
88,00
CALCIUM HARDNESS (%)REMOVAL
87,00
86,00
85,00
84,00
83,00
82,00
81,00
80,00
NaOH
40mg
80mg
120mg
160mg
85,33
85,78
86,67
86,67
The graphs show a general increase in hardness removal for both total and calcium hardness as
the NaOH amount increases. This is due to the increase in cationic exchange sites resulting from
an increase in NaOH. The efficiency removal for calcium hardness is greater than that for total
hardness because total hardness encompasses both calcium and magnesium hardness. For every
molecule of sodium released from the zeolite resin, two calcium molecules are adsorbed onto the
zeolite thus removing hardness.[1,6] So as NaOH concentration is increased more calcium ions are
removed from solution. There exists a definite increase of removal efficiency between the 40mg
and 80mG NaOH dosage.
a)
EFFECT OF RESIDENCE TIME
TOTAL HARDNESS REDUCTION
100,00
90,00
80,00
TOTAL HARDNESS % REMOVAL
70,00
60,00
50,00
40,00
30,00
20,00
10,00
-
1 hr
2 hrs
3 hrs
4 hrs
5 hrs
6 hrs
7 hrs
8 hrs
sample A
10,91
18,22
23,31
50,99
63,64
65,71
65,97
66,13
sample B
12,68
18,83
24,68
53,64
62,60
64,94
66,56
66,59
sample C
13,64
19,61
27,16
55,68
65,29
65,42
66,36
67,25
sample D
14,03
20,54
28,75
56,04
66,47
66,37
69,74
70,52
CALCIUM HARDNESS REDUCTION
100,00
90,00
80,00
CALCIUM HARDNESS % REDUCTION
70,00
60,00
50,00
40,00
30,00
20,00
10,00
-
1 hr
2 hrs
3 hrs
4 hrs
5 hrs
6 hrs
7 hrs
8 hrs
sample A
18,10
36,39
52,64
72,89
80,11
82,64
83,11
83,79
sample B
19,62
38,09
54,82
72,89
80,75
83,11
84,46
85,54
sample C
19,94
38,42
55,00
73,78
83,67
85,56
86,01
86,56
sample D
20,98
40,33
59,79
74,44
84,89
86,00
86,23
87,78
The general trend for both total and calcium hardness shows an increase in hardness removal as
the residence time increases. The contact time was observed from 1hour to 8 hours. The hardness
removal percentage approached equilibrium at 7 hours for both total and calcium hardness. After
7 hours, any increase in time results in slight increase of hardness removal. This is the period
when most sodium cationic exchange sites have been occupied by hardness ions. In addition to
this another factor are the repulsive forces between hardness ions on the solid zeolite phase and
in the liquid phase which create difficultness for the hardness ions to occupy remaining vacant
surface sites.
a) EFFECT OF ZEOLITE MASS SAMPLE
TOTAL HARDNESS REDUCTION
70,00
60,00
TOTAL HARDNESS % REDUCTION
50,00
40,00
30,00
20,00
10,00
-
1g
2g
4g
6g
8g
10g
sample A
15,45
22,82
31,04
41,30
56,01
60,73
sample B
15,72
24,82
32,30
43,21
57,14
61,69
sample C
16,84
25,08
33,69
44,95
57,79
62,34
sample D
17,62
26,08
34,08
45,86
58,44
63,73
There is overall increase in hardness removal as zeolite mass sample is increased. Increase in
sample mass increases the zeolite cationic exchange sites therefore increasing removal efficiency
with mass increase.[7] The optimum mass sample from experimental results was 8g/100ml. a
further increase in zeolite sample mass used yielded little change in removal efficiency increase
hence the selection of 8g/100ml as the optimum sample time.
CALCIUM HARDNESS REDUCTION
90,00
80,00
CALCIUM HARDNESS % REDUCTION
70,00
60,00
50,00
40,00
30,00
20,00
10,00
-
1g
2g
4g
6g
8g
10g
sample A
26,00
44,22
58,67
68,89
71,00
82,44
sample B
28,67
45,33
58,89
68,89
72,89
83,11
sample C
29,67
47,67
60,78
69,56
73,67
84,67
sample D
31,11
48,89
61,00
69,78
74,89
85,33
5.0: CONCLUSION
The experimentation done in this research study achieved positive results in as far as hardness
removal is concerned. Percentage hardness removal efficiencies of 67% and 83% were achieved
for total and calcium hardness respectively. The following deductions were also made:
i.
The optimum ratios for preparing the fly ash zeolite are:
 pH of 7.2
 NaOH dosage of 120mg/ 200g of ash
ii.
The optimum water treatment conditions were:
 pH of 7.2
 Residence time of 7hrs
 8g mass of zeolite
iii.
An agitation mechanism of the zeolite and water sample is to be employed for the
scaled up softeners as it increased the rate of mass transfer thus increasing the
efficiency of the process.
In addition to this though not investigated, is the benefit of investigating the regeneration of the
spent zeolite which is recommended for further study. Further analysis focussing on magnesium
ions and their removal from borehole water is also recommended.
ACKNOWLEDGEMENTS:
The authors would like to extend their sincere gratitude to the Zimbabwe Power Company
Bulawayo station for allowing the research experiments to be carried out on their premises as
well as providing the necessary resources to carry out the research study.
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