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2.4.1.3 Influence of temperature
The temperature is an important factor in the kinetics of solid degradation affecting the metal
leaching rate. In order to improve the extraction efficiency, higher temperature values are
desirable and for this reason wide ranges of temperatures have been studied in different
experimental designs. Thermal treatment of slags includes approaches such as high
temperature leaching using various leaching agents at temperatures ranging 24 – 100°C
(Anand et al., 1980; Basir & Rabah, 1999; Banza et al., 2002; Carranza et al., 2009; Yang et
al., 2010; Ahmed et al., 2012), thermal aging of slag moisture combined with leaching at 20 –
250°C (Deng & Ling, 2007) as well as slag roasting (30 - 800°C) following leaching (Anand
et al., 1981; Sukla et al., 1986; Altundoǧan and Tümen, 1997; Arslan & Arslan, 2002;
Nadirov et al., 2013).
The most common conclusion from experiments applying thermal treatment is that
temperature enhances both the metal leachability for slag pre-treatment as well as their
leaching efficiency. However, in some cases thermal activation was noted to be a catalysing
CHAPTER 2: COPPER METALLURGICLA SLAGS- CURRENT KNOWLEDGE AND FATE:
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factor only to a certain level of temperature elevation due to evaporation of the leaching
agents at higher temperatures (Sukla et al., 1986; Tshiongo et al., 2010; Nadirov et al., 2013).
Anand et al. (1980) observed that copper may be efficiently leached by ferric chloride even at
ambient temperature reaching values of 64% (25.8 g/kg), but increased temperature facilitated
its leaching, promoting Ni and Co leaching as well. Basir and Rabah (1999) reported thermal
activation had a favourable effect on leaching when hydrochloric acid, sulfuric acid and
ammonium hydroxide were used as extractants. Banza et al. (2002) studied the effect of
temperature in the range of 24-80°C and observed a significant increase of the metal
extraction efficiency with increased temperature, especially when temperature rises up to
60°C. Banza et al. (2002) achieved high levels of metals recovery up to 60% (8.6 g/kg) Cu,
90% (6.5 g/kg) Co, 90% (80.1 g/kg) Zn and 90% (186.3 g/kg) Fe within 2 hours at 80°C.
Carranza et al. (2009) indicated a similar temperature range (25-50°C) where copper
extraction by ferric sulphate was noticeably improved (30% more Cu). Enhanced Cu
extraction was obtained by increasing the temperature to 70°C using sulfuric acid as leaching
agent, whereas the optimum temperature for Zn was already achieved at 35°C (Ahmed et al.,
2012).
Thermal aging of an acid moisture following water leaching proposed by Deng & Ling (2007)
was found to give satisfactory extractions of metals. In this case, the increased aging
temperature was concluded to be the favourable factor for Co (100°C) as well as Cu and Fe
(250°C) extraction.
Slag roasting as thermal pre-treatment of slag following leaching gives a possibility to extract
high quantities of metals. Anand et al. (1981) performed reduction roasting following ferric
chloride leaching. This approach has a higher importance for Co and Ni present in slags in the
oxide form, whereas it may slightly decrease Cu leachability. Roasting/leaching done by
Sukla et al. (1986) has shown a greater extraction efficiency along with increasing roasting
temperature. However, the authors pointed out that increasing the temperature is favourable
only to the certain level, because temperatures above 200°C may decrease the metal
extraction efficiency due to the evaporation of H
2
SO
4
. Altundoǧan and Tümen (1997) who
proposed ferric sulphate as roasting agent noticed a positive effect of elevated temperatures on
the metal extraction efficiency even until 500-550°C. This roasting temperature may give as
much as 95% (24.7 g/kg) for Cu during 0.5 h leaching (Altundoǧan and Tümen, 1997). Arslan
& Arslan (2002) reported slightly lower Cu extraction efficiencies (88%) (23.2 g/kg) at 150°C
using sulfuric acid as roasting agent. However, elevated roasting temperatures (250°C) allow
to extract even 100% (26.4 g/kg) of Cu. Furthermore, results of the thermal treatment using
ammonium chloride following water leaching also confirmed higher temperatures as the
factor promoting metal extraction. A temperature of 320°C allows to extract 89.7% (19.7
g/kg) of Cu during 2 h leaching, while maintaining the Fe extraction efficiency on a low level
(35%) (127.4 g/kg) (Nadirov et al., 2013).
CHAPTER 2: COPPER METALLURGICLA SLAGS- CURRENT KNOWLEDGE AND FATE:
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