Journal of Hazardous Materials 181 (2010) 399–404
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Journal of Hazardous Materials
journal homepage: www.elsevier.com/locate/jhazmat
Formation of copper aluminate spinel and cuprous aluminate delafossite to
thermally stabilize simulated copper-laden sludge
Ching-Yao Hu
a
, Kaimin Shih
b,∗
, James O. Leckie
c
a
School of Public Health, Taipei Medical University, 250 Wu-Xin Street, Taipei City, 110, Taiwan, ROC
b
Department of Civil Engineering, University of Hong Kong, Rm. 5-26, Haking Wong Building, Pokfulam Road, Hong Kong
c
Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305-4020, USA
article info
Article history:
Received 19 November 2009
Received in revised form 5 May 2010
Accepted 6 May 2010
Available online 12 May 2010
Keywords:
Sludge
Copper
Spinel
Alumina
Kaolinite
abstract
The study reported herein indicated the stabilization mechanisms at work when copper-laden sludge is
thermally treated with -alumina and kaolinite precursors, and evaluated the prolonged leachability of
their product phases. Four copper-containing phases – copper oxide (CuO), cuprous oxide (Cu
2
O), copper
aluminate spinel (CuAl
2
O
4
), and cuprous aluminate delafossite (CuAlO
2
) – were found in the thermal
reactions of the investigated systems. These phases were independently synthesized for leaching by 0.1 M
HCl aqueous solution, and the relative leachabilities were found to be CuAl
2
O
4
< CuAlO
2
≪ Cu
2
O < CuO.
The sintering condition and formation mechanism employed to stabilize copper into CuAl
2
O
4
and CuAlO
2
are extensively discussed here. With a 3 h of short sintering, it was found that CuAl
2
O
4
could be effectively
formed between 850 and 950
◦
C by the -alumina precursor. Although kaolinite had a lower incorporation
capability than -alumina, it was found to transform a considerable amount of copper into CuAl
2
O
4
between 950 and 1000
◦
C. At higher temperatures, CuAlO
2
was produced only in the -alumina system as
the occurrence of Cu
2
O-cristobalite solution in the kaolinite system precluded the production of CuAlO
2
.
The hypothesis that the spinel formation mechanism has two stages was supported by the results of the
changing Cu/Al mole ratio in the system, and the rate-limiting step was identified as the diffusion process
in the second stage.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Hazardous copper ions can cause severe damage to the stom-
ach, intestines, and liver. They can also cause anemia in humans [1]
and are highly toxic for some aquatic biota [2–4]. The various pro-
duction processes relating to circuit board printing, metal surface
treatment and mining operations comprise the primary sources for
the discharge of hazardous copper. In Taiwan, for instance, over
100,000 tons of copper-laden sludge is produced annually, and
because copper is highly leachable that sludge increases the risk
of water and soil pollution.
A conventional method of treating hazardous metal sludge is to
stabilize/solidify (SS) solid waste by using cement prior to land-
fill. This method is attractive because of its low initial cost [5].
However, a large amount of cementitious material is required to
make solidified waste sufficiently strong to prevent the leaching
of hazardous metals from broken products, and this significantly
increases the volume of cement needed for landfill [6–8]. It has also
been found that the metal incorporation mechanisms in cement
∗
Corresponding author. Tel.: +852 2859 1973; fax: +852 2559 5337.
E-mail address: kshih@hku.hk (K. Shih).
products are not unconditionally stable and that the reactions of
incorporation may be reversible. Many observations have demon-
strated that these cement products have been unable to prevent
the leaching of heavy metals in acidic environments, i.e., at pH lev-
els of less than 4.0 [9,10], or under conditions of aggressive CO
2
attack [5]. Moreover, the cost of disposing of cemented sludge that
contains hazardous metals has significantly increased due to the
introduction of stringent regulations that limit the number of land-
fills qualified for receiving such waste. Therefore, alternative heavy
metal stabilization strategies that depend on irreversible stabiliza-
tion mechanisms and enable the reuse of stabilized products are
needed.
The stabilization of metal sludge via thermal treatment has the
potential to convert hazardous metal-laden sludge from the waste
stream into reusable products. The goal is to significantly reduce the
metal leachability of products via an irreversible transformation of
metals into their mineral phases after thermal treatment [11–18].
Shih et al. successfully stabilized nickel by sintering its oxide with
alumina (Al
2
O
3
), hematite (Fe
2
O
3
), and kaolinite (Al
2
Si
2
O
5
(OH)
4
)
[11,17,18]. They found that the sintering of simulated sludge
at 1000
◦
C with alumina and at 600
◦
C with hematite, respec-
tively, resulted in the formation of nickel aluminate spinel and
nickel ferrite spinel. The long-term leachability of nickel dropped
0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jhazmat.2010.05.024