09-11-2016, 11:35 AM
Recycling of Waste Paper Sludge in Cements: Characterization and Behavior of
New Eco-Efficient Matrices
1467517180-RecyclingofWastePaperSludgeinCements18492.pdf (Size: 1.12 MB / Downloads: 59)
Introduction
The pulp and paper industry, in Europe, generates 11 million tons of solid waste each year
(Monte et al., 2009). Paper waste covers a diverse range of non-hazardous waste streams,
prominent among which are different types of sludge, boiler ash, combustion furnace ash
and organic and inorganic rejects. Manufacturing processes to produce new paper from the
deinking of recycled paper account for 70% of these waste products.
Following its reception, sorting and storage, the recycled paper is transformed into an
aqueous suspension of fibers, while inappropriate materials are eliminated in different
cleaning processes. Following this initial treatment, the resultant paper sludge is subjected
to deinking in a froth flotation process, which produces waste known as de-inked sludge.
This waste sludge is fundamentally composed of water, fiber, ink and a mineral load. In
addition, various paper manufacturing processes have water treatment plants that generate
sludges with high humidity contents.
The deinked paper sludge and the sludge from the water treatment process have a high
humidity content (≈ 50%), and are roughly composed of organic material with their origin in
paper fibers (≈ 25%) and mineral loads such as calcium carbonate, kaolin, talc and titanium
oxide (≈ 25%). A similar composition highlights the wealth of energetic and mineral
resources saturating the paper sludge. Thus, the most advanced techniques for the use of
paper sludge are intended to take full advantage of the saturated biomass and the recovery
of the mineral constituents in the inorganic fraction.
The most common options for the processing of paper industry sludge range from their
exploitation for agricultural purposes, composting, or use as a primary material in the
manufacture of ceramics and cement (Moo-Young & Zimmie, 1997; Ahmadi & Al-Khaja,
2001; Lima & Dal Molin, 2005; Conesa et al., 2008), to energy recovery in biomass boilers or
fluidized bed systems. Thus, the Dutch CDEM process (International Patent, 2006)
represents a pioneering recovery system, where the paper sludge is treated at temperatures
of around 730ºC, in a fluidized bed combustion system, so as to activate the latent
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302 Integrated Waste Management – Volume II
pozzolanic properties of its mineral content. The CDEM process was industrialized after the
pioneering work of research groups led by Prof. Pera (Pera & Amrouz, 1998), which
demonstrated that controlled calcination of the deinked sludge produces a highly reactive
pozzolanic material, within a temperature range of between 700 and 750ºC.
On the basis of the scientific knowledge presented earlier, a team of Spanish researchers led
by Dr. Frías, has been conducting in-depth research over the past decade into the scientific,
technological and environmental aspects of obtaining active admixtures from the calcination
of paper sludge and its behavior in cement and mortar.
2. Waste paper sludge and its activated products
2.1 Nature of the raw waste and activation process
The characteristic composition of this industrial waste is a mixture of organic material (nonrecovered
cellulose) and inorganic materials (principally, kaolinite and limestone), normally
used as loadings in the manufacture of paper.
An example of the chemical and mineralogical composition of this type of waste is
presented in Table 1. The characterization of this dry material is provided by the Spanish
paper manufacturer Holmen Paper Madrid, S.L, which uses 100% recycled paper as the raw
material. X-Ray Fluorescence (XRF) confirms that the principal oxides are CaO, SiO2 and
Al2O3, the sum of which exceeds 43% of the total mass. The high Loss on Ignition (LOI) in
these waste products, at around 54%, should be underlined, due to the presence of organic
material, kaolinite dehydroxylation and the decarbonation process of calcite. These values,
for guidance only, vary in accordance with the type of paper, its origin, the percentage of
recycled paper used as primary material, the loadings, and the type of process etc. With
respect to its mineralogical composition, it is worth highlighting the presence of cellulose
residue (about 32%, determined according to the results of XRF and XRD), as well as the
presence of calcite and kaolinite content in a ratio of 3.3 (Frías et al., 2010). This value is
above those in other research works that report ratios of under 2, even for samples from the
same paper manufacturing process (Pera & Amrouz, 1998; Frías et al., 2008a). The variation
in the composition of this industrial waste is therefore confirmed.
2.2 Properties of the activated products
Knowledge of the chemical, physical, mineralogical and pozzolanic properties that
determine the behavior of Portland cements prepared with activated wastes in the form of
active additions represents a key point for the evaluation of their viability.
2.2.1 Physical properties
Laser diffraction granulometry confirms the presence of particle sizes of less than 90
micrometers. The distribution density curves show 2 maximums located at 40 and 4
micrometers. The BET surface area varies between 7 and 8 m2/g, for original activated
sludge, a much higher value than that found for a cement type I 42.5 R (<1 m2/g), reaching
values of around 12-13 m2/g for activated paper sludge that is ground down to particle sizes
of less than 45 micrometers (Ferreiro, 2010).
The different coloration between the raw paper sludge and the activated product is also
worth mentioning (Fig.1). Whereas the former presents a grayish coloring due to the
deinking process, the latter shows a white color
2.2.2 Chemical composition
In a similar way to the processes described for basic paper sludges, the products yielded by
thermal activation are formed principally of silica (20-30%), lime (34-45%), alumina (13-20%)
and magnesia (2-3.5%). The remaining oxides are present in amounts of less than 1%. The
chemical values increase with the intensity of the activation conditions, as a consequence of
the reduction in loss on calcination. These results are in accordance with those obtained by
Bai (Bai et al., 2003), but differ from those indicated by Toya (Toya et al. 2006).
2.2.3 Mineralogical and morphological composition
The mineralogical composition of the activated sludge from the most labile (500ºC for 2
hours) to the most drastic (800ºC for 2 hours) conditions reflects the changes undergone in
the different minerals due to heating. The paper sludge calcined at 500ºC for 2 hours is
composed of talc, kaolinite, illite, dolomite, calcite and quartz. As the temperature increases
(550ºC/2 hours), the kaolinite is transformed into metakaolinite. This compound is detected
by SEM, as it is not a crystalline material (Fig. 3). The talc and quartz remain unaltered in the
range of temperatures under study. In contrast, the dolomite is transformed at 550ºC/2
hours and the calcite disappears at 800ºC/2 hours, as a result of the decarbonation of those
minerals. The illite undergoes a transformation process at 800ºC/2 hours. The appearance of
portlandite is notable at 650ºC/5 hours or more as a consequence of the exposure of the
paper sludge to environmental humidity, while the formation of dicalcium silicate
(bredigite) is detected at 800ºC or more.
The behavior of binary and ternary blended cement prepared with
thermally activated paper sludge
3.1 Scientific aspects
3.1.1 Reaction kinetics in binary cements with the addition of 10% activated sludge
In general, the kinetics of pozzolanic reactions depends on various chemical, physical and
mineralogical factors. In a study of the influence of the activation conditions on the hydrated
phases, percentages of 10 and 20% cement were replaced in this study, which gave similar
results. For example, the mineralogical behavior is described here over the reaction time in
prismatic specimens (1x1x6 cm) of paste cement prepared with the addition of 10% paper
sludge calcined at 700ºC/2h.
XRD and SEM/EDX techniques were used to perform the kinetic study of the reaction, so as
to semi-quantify the formation of hydrated phases and the development of their
morphologies with the reaction time. The XRD results are provided in Table 2, where the
appearance of allite, portlandite, calcite, calcium aluminate hydrates (C4AH13), and LDH
compounds (or compounds of double oxides, at times referred to as hydrotalcite-type
compounds) were detected; the last three materials being the most stable over longer
periods.