CHARACTERIZATION OF SINTERED POROUS ACTIVATED
CARBON BLOCKS FOR DRINKING WATER FILTRATION
I. CVIJOVI], O. DIM^I]* and D. BO@I]**
Department of Material Science, Institute of Nuclear Sciences „Vinča“, P.O. Box 522,
11001 Belgrade, Serbia, ivanac@vin.bg.ac.yu
*Technocon filter d.o.o., Bulevar Mihaila Pupina 165a, 11070 Belgrade, Serbia,
olivera.dimcic@tcfbeograd.com
**Department of Material Science, Institute of Nuclear Sciences „Vinča“, P.O. Box 522,
11001 Belgrade, Serbia, dbozic@vin.bg.ac.yu
ABSTRACT
The aim of present research was to achieve a hot-pressing treatment, with
defined process parameters, which will insure production of the sintered
activated carbon (AC) cylindrical blocks for drinking water filtration.
Generally, our aim was to obtain clean and uncontaminated water, refined
from high chlorine content and substances responsible for the water aroma,
colour or taste. Conducted research included AC and polyethylene powders
caracterization, mixed powders hot-pressing treatment and obtained porous
compact blocks characterization and analysis. Scanning electon microscopy
(SEM) was used for microstructural characterization, while BET analysis
was conducted in order to determine specific surface area of obtained porous
compacts. Binding material content and hot-pressing pressure as a function
of water flow duration through the porous cylinder were also determined.
Results obtained during this research should contribute to better
understanding of the AC consolidation procedure in order to expand
application of activated carbon products used in the water refinement
purposes.
Keywords: Sintered AC blocks, Drinking water filtration, Hot-pressing,
SEM, BET analysis
Introduction
Activated carbon (AC) is one of the most commonly used materials for chemical
filtration, when water pollution is concerned [1]. For the water filtration AC can be used in the
form of low packed powder, granulates, or in the shape of sintered compacts, blocks [2]. In
comparison with the granular AC, compacted blocks are more effective in water filtration because
of their ability to adsorb higher concentrations of various harmful substances (e. g. chlorine,
pesticides, industrial chemicals and various organic chemicals), uniformity of their pores,
dimensional stability and microbiological filtration ability. Also, water pressure drop is ten times
smaller in the compacted then in the granular AC filters, water flow through a compacted filter is
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uniform because water flows in radial direction and not in axial like in the case in the granular AC
filters, the AC compacts can be used in differently positioned filter constructions (low packed AC
powders can be used only in vertical constructions) and last, but not the list, compacted AC filters
have longer work life.
The aim of the present research was to achieve a hot-pressing treatment, with defined
process parameters (i. e. temperature, pressure and hot-pressing duration), which will insure
production of high-quality AC compact filters for drinking water refinement. To fulfill this
objective, an extensive characterization of obtained AC compacts was carried out as a function of
hot-pressing parameters.
Experimental procedure
Activated carbon powder, supplied by Amersack Corporation, Guatemala, and
polyethylene powder, supplied by HIP – Pančevo, Serbia, were used in order to produce hotpressed compacts. Sieving of the comercial activated carbon powders was undertaken and in that
purpose sieve size range from 63 µm do 500 µm was used. Powder fraction with particles size
range from 180 µm to 400 µm was choosen for the experimental proccessing. In this way particals
size range was narrowed, consequently enabling formation of compacts with open pores of similar
size. Afterwards, AC and binding material powder mixture was prepared in order to obtain porous
AC blocks using the hot-pressing treatment. Mixing of AC and polyethylene powders was
conducted in the cylinder mixer supplied by Netzsch, Germany. Mixer rotational speed was 70
rev/min. Polyethylene content in the powder mixture was in the range from 10 to 25 wt.%, while
blending duration was varied from 15 to 60 min. When preparation of homogeneous powder
mixture was completed, mixture was poured out in the hot-pressing tool, manufactured from the
tool steel in the facilities of the company Tehnocon filter d.o.o., Serbia, with construction
dimensions defined in regard to the requested cylindrical AC blocks dimensions. In order to define
optimal hot-pressing parameters, temperature control of the hot zone in the hot-pressing tool was
carried out during compacting procedure.
Caracterization of the supplied AC and polyethylene powders was conducted and it
included screen analysis, microstructural characterization and powder quantitative analysis. A
Philips XL30DX41 scaning electon microscope (SEM) and an image analyzer were used in that
purpose. After hot-pressed compacts were manufactured, analysis of their density/porosity, pore
dimensions, permeability, as well as water flow rate through the cylindrical AC compact were
determined in order to correct some of the process parameters of the preparation treatment
(temperature and duration of AC and binding material annealing, mixing duration and mixing rate
of the particles, composition of the binding material, mixture quantity) and final hot-pressing
treatment (hot-pressing pressure, temperature and duration).
Results and discussion
Results of the SEM analysis showed that AC particles are irregularly shaped.
Submicronic cavities, the main reason for the AC exceptional adsorption power, were detected in
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their microstructure. In contrast, individual polyethylene particles (5 - 15 µm in size) are shown to
be rounded and spherical in shape.
Powder quantitative analysis showed that the best binding material distribution in the
AC powder was achieved with 30 min blending of the 15 wt.% polyethylene powder mixture (Fig.
1). From the SEM micrograph, shown in Fig. 1a, one can observe the presence of the fine
polyethylene particles located between AC particles Areas without polyethylene particles could
not be detected (erosion of the porous cylindrical compacts was avoided) and the time needed for
compacts formation is proved to be extremely short.
Using the polimerization hydraulic press, produced by Buehler Ltd., USA, formation of
the porous cylindrical compacts (D = 20 mm, H = 20 mm) was conducted in air. Hot-pressing
temperature was kept between 120 °C and 140 °C, processing duration at the selected temperature
was between 1 and 30 min, while hot-pressing pressure was in the range from 10 to 25 MPa.
Powder mixtures with various polythylene contents (from 10 to 22.5 wt.%) were used. Although
all these listed parameters can influence the powder mixture hot-pressing processing, most
influential process parameter is certainly hot-pressing temperature and because of that defining of
the optimal process temperature was of the prime concern for our research. Achieving of the
narrow temperature range, in which polyethylene is in its plastic phase i.e. form in which can be
used for binding of the solid AC particles, was one of our main objectives. Several experimental
series were conducted during which hot-pressing temperature value was maintained in the range
from 125 oC (longer contact between particles) to 140 oC (shorter contact between particles),
depending on the hot-pressing treatment duration. Finally, it was determined that optimal values of
hot-pressing temperature and duration are 125 oC and 30 min, respectively. Achived
microstructure (Fig. 1b) was characterized with homogeneously distributed polyethylene, which
bonded AC particles in the entire compact during its liquid state, under the influence of the hotpressing temperature, duration and pressure.
200 μm
500 μm
(a)
(b)
Fig. 1. SEM micrographs of (a) the activated carbon and polyethylene mixture (15 wt.%
polyethylene) after 30 min mixing treatment and (b) the hot-pressed activated carbon cylindar
microstructure.
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Two other hot-pressing parameters (pressure and binding material content) had similar
influence on the compacts density/open porosity. Namely, increase of the hot-pressing pressure or
binding material content led to compacts density increase and open porosity decrease. Almost
identical i.e. linear dependence of the density/open porosity on the hot-pressing pressure/binding
material content was observed (Fig. 2).
(a)
(b)
Fig. 2. (a) Density/open porosity and binding material content dependence and (b) density/open
porosity and hot-pressing pressure dependence.
In order to utilize obtained results for the development of the porous compacts forming
technology, it was necessary to conduct two more laboratory tests. First of all it was crucial to
determine dependence of the water flow duration through the porous cylinder on the polyethylene
content, as well as on the hot-pressing pressure (Fig. 3a). Also, it was of a great importance to
determine AC and cylindrical compact specific surface area, using the BET (Brunauer, Emmet and
Teller) analysis (Fig. 3b). Although lowest values of the above mentioned parameters (10 wt.%
polyethylene mixture and 10 MPa hot-pressing pressure) resulted in the best physical
characteristics of the cylindrical compacts (lowest density i.e. highest open porosity) they could
not be used as optimal process parameters because of the high erosion of the compacts formed
under these conditions. Finally, it was concluded that optimal process parameters values, which
can be used in order to obtain compacts with the required mechanical strength combined with the
maximal porosity, were 15 wt.% polyethylene mixture and 15 MPa hot-pressing pressure, since
cylindrical compacts with higher binding material content formed under higher pressure were
characterized with lower value of the water flow through the cylinder and lower adsorption
potential.
After detailed analysis of the results obtained in the laboratory conditions, necessary for
the projecting and production of the AC compact filters, the AC cylindrical compacts test series
was produced. Chemical analysis of the unfiltered water and water filtered through the obtained
compacted AC cylinders was undertaken and it was concluded that production of the porous
compacted AC cylinders was successful and that prime objective of this research was achived.
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(a)
(b)
Fig. 3. (a) Water flow duration dependence of binding material content and hot-pressing pressure
and (b) adsoption curves for specific area calculation.
Conclusions
Optimal hot-pressing parameters were successfully determined and it was found that
compacting of the 15 wt.% polyethylene + 85 wt.% AC powder mixture was enabled applying the
pressure of 15 MPa for 30 min at 125 °C. Results of this research showed that it is possible to
produce homogenous commercial AC compacted filters using the simple compacting procedure,
such is one-sided hot-pressing.
Acknowledgements
This research was financially supported by the Ministry of Science of the Republic of
Serbia through the Project No. 142027.
References
[1] L. Wagenet, K. Manel, M. Sailus, Home Water Treatment, Northeast Regional Agricultural
Engineering Service, Cooperative Edition, Ithaca, NY, 1995.
[2] www.kxindustries.com
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