Also the temperature at which leaching is performed has a great effect on recoveries of valuable metals. This has been studied by Aydogan et al. (2006), Córdoba et al. (2008) and Koleini et al. (2011). As a result of all the investigations an increase in temperature was concluded to increase the recoveries. However an optimum temperature should always be found out since increasing temperature also means an increase in heating costs. Economic aspects and sustainable development should always be taken into consideration and hence operating at high temperatures might not be the best way in terms of economy.
Aydogan et al. (2006) studied the kinetics of dissolving a chalcopyrite concentrate in acidic potassium dichromate. The concentrate was obtained from a flotation circuit and contained a complex ore CuFeS2-PbS-ZnS. Leaching was done at temperatures from 50 °C to 97 °C and the temperatures in each test were kept constant by the use of a thermostatically controlled water bath. At a temperature of 50 °C the amount of extracted copper was 54 % whereas at 92 °C it was increased to 82 %. The reaction has however not reached the state of equilibrium and hence comparison of the recoveries of copper at equilibrium can not be made.
The increase in temperature can thus only be shown to speed up the kinetics of the reaction.
Córdoba et al. (2008) also concluded in their study on the leaching of chalcopyrite using ferric ions that an increase in temperature increases the recovery of copper.
In four different temperatures between 35 and 68 °C and after 13 days of leaching they experienced an increase in the extraction of copper from less than 3 % to more than 80 %, respectively. In these experiments the reactions done at 57 °C and 68 °C seemed to reach equilibrium whereas the two reactions at 35 °C and 46
°C were still under linear growth after 13 days of leaching. Equilibrium at the two higher temperatures was reached approximately after seven days with a temperature of 68 °C and after ten days with a temperature of 57 °C.
Also Koleini et al. (2011) came to a conclusion that increasing temperature increases copper recovery. The chalcopyrite concentrate was leached in sulfuric acid in the presence of pyrite in three different temperatures, 48 °C, 68 °C and 85
°C. A copper recovery of higher than 80 % was achieved at the highest temperature after 24 hours of leaching whereas the recovery with a temperature of 68 °C was about 15 percentage points less. Both of these higher temperature recoveries were more than four times greater than the recovery obtained in 48 °C leaching. The recovery of copper showed a linear dependency with respect to temperature after 24 hours of leaching.
Baláž et al. (2000) studied the effect of temperature on the leaching behavior of a pentlandite concentrate. Also in this study an increase in temperature gave a higher yield of the compound to be extracted. The main components in the concentrate were pentlandite, chalcopyrite, pyrrhotite (Fe1-x(x=0-0.17)S) and pyrite.
First the concentrate was leached in water after which leaching in Fe2(SO4)3
solution was performed. Leaching was done at two different temperatures, 45 °C and 90 °C, for an as received concentrate sample and a ground (referred by Baláž et al. (2000) as a mechanically activated) sample. Grinding was carried out in order to accelerate the leaching process (Maurice and Hawk, 1998). When grinding a sample the surface area of the sample increases which leads to an increased reactivity at the surface and to structural changes in the sample. For example for a pentlandite sample, grinding increases the surface area and
decreases the amount of the crystalline phase hence increasing the degree of amorphization.
In the study made by Baláž et al. (2000) nickel, copper and cobalt were extracted.
Recoveries were presented for the second step of the leaching reaction, which was the leaching in Fe2(SO4)3. The recoveries at the beginning of this second step were not zero since some water soluble sulfides were formed in the grinding stage.
These water soluble sulfides were leached already in the first leaching stage. The recoveries presented graphically by Baláž et al. (2000) are shown in Table II.
TABLE II Recovered values at two different temperatures from a sample as received and a sample after mechanically activating (grinding) it for 60 min after 120 min leaching in Fe2(SO4)3. Data read from graphs presented by Baláž et al. (2000).
Sample As received Grinded Temperature (°C) 45 90 45 90 Recovered Ni (%) 6 13 26 73 Recovered Cu (%) 9 19 33 100 Recovered Co (%) 9 36 30 89
As shown in Table II, the recovered values of valuable constituents are higher for the ground sample when compared to the as received sample. A higher temperature also gave a significantly higher recovery in the case of mechanically activated samples.
The effect of temperature on the rate of reaction can be explained by the Arrhenius equation shown in equation 1.
RT EA
Ae
k= − (1)
In which k is the reaction rate constant, A the pre-exponential factor, EA the activation energy, R the universal gas constant and T the absolute temperature in Kelvin. Activation energy does however not reveal any significant information on the kinetics or mechanisms of dissolution (Crundwell, 2013). It only gives you two values: if the activation energy, EA, is lower than 20 kJ/mol the reaction is
diffusion controlled in the aqueous phase and if it is above 40 kJ/mol the reaction is chemically controlled. However even these values are generalized and changes might occur depending on temperatures used (Córdoba, 2008).
Activation energies were studied by several authors (Biswas et al., 2014; Córdoba et al., 2008; Kaplun et al., 2011; Li et al., 2013; Kimball et al., 2010; Nicol et al., 2010; Sokić et al., 2009; Vračar et al., 2003) in order to get suggestions for the controlling factor of the leaching process. Vračar et al. (2003) for example used activation energy to prove that the leaching of copper (I) sulfide with sulfuric acid and added sodium nitrate was chemically controlled (the value of activation energy obtained was 60 ±0.7 kJ/mol). High activation energies would indicate a need for high temperatures since the bonds in the mineral need to be broken down in order for leaching to occur. Hence finding out activation energies gives some direction on how high temperatures are optimal.
As a conclusion of the studies shown above, an increase in temperature seems to increase the amount of valuable constituents recovered and speed up the kinetics of the reaction. In the study by Córdoba et al. (2008) the equilibrium concentration of dissolved copper increased with increasing temperature, but for studies made by Aydogan et al. (2006), Baláž et al. (2000) and Koleini et al.
(2011) no conclusions on the effect of temperature on the equilibrium concentration could be made. The reaction kinetics were however shown to speed up by increasing the temperature. Note should be made to the fact that increasing temperature is not always the most economical way to increase the recovery. An optimum temperature should be found out instead of using the temperature giving the highest recovery.