Li-ion batteries (abbreviated as LIB) are a type of electrochemical batteries that are popular for electronic devices all around the world given its high energy density, high capacity, low discharge, good performance and light weight  -  . While there are several types of Li-ion batteries, the most used in electronics (like cellphones) is the one based on lithium cobalt oxide (LiCoO2) because of its high energy density. While it possesses high safety risks  , it is still the best option for consumer electronics which makes the volume of batteries made quite huge, so the volume of waste generated by them is equally high. This, along with some countries wih poor or non-existent lithium deposits, makes recycling an interesting proposal both environmentally and economically, with leaching being the most used method for the recovery of the metals that are part of the aforementioned batteries.
One of the most used leaching processes uses acids such as sulfuric acid, H2SO4, which are not too healthy for the environment, so the search for alternatives of leaching agents is an active research topic      . Organic agents are an alternative to acids given that their enviromental impact is less hazardous and can be easily disposed after being used. The only used in this experiment is sodium citrate, NaH2(C3H5O(COO)3), which is the sodium salt in the citrate acid found in the cells of the human body. Ultrasound has become an increasingly used method along with leaching because it is considered that it increases the leaching efficiency and the yield of products  . Thus, this process increases the yielded metal residues while reducing the time and amount of leaching agents used to obtain them.
Many works had mentioned high conversions of Co and Li obtained mixing two reagents, citric and ascorbic acids  , citric, acetic and oxalic acids with hydrogen peroxide  . Other authors have shown good results as well using citric acid with hydrogen peroxide  ; however, the use of other reagents has not increased the Co and Li conversion substantially. Nevertheless, the temperature and the ultrasonic agitation have already improved the results. In the present work the use of the only citrate acid in combination with the ultrasonic process presented.
Batteries were gathered from collection and recycle centers, then they were then disassembled and their components separated in unwanted and useless residues, and useful residues for leaching. The useful residues were prepared for X-Ray Diffraction (XRD) analysis to know which elements were present, while the Atomic Absorption Spectrometry (AAS) technique was employed for characterization. For the leaching process, was used an ultrasonic bath of 3 liter, with temperature control, the concentrations of the organic agent were established in 0.5 M, 1 M and 1.5 M; then, according to the compounds, the leaching process was done changing temperature from 25˚C, to 50˚C and 60˚C during the process. Ultrasound frequency was varied using 20 KHz, 30 KHz and 40 KHz. Finally, it was determined the efficiency of the process, quantifying the wanted metal residue yielded after leaching related to the initial amount in the batteries and the agents used.
3. Results and Comments
From the several combinations of the three variables proposed, it was found three specific combinations to be the most optimum for lithium recovery, as is shown in Figure 1. As one can see, the optimal combination of variables for lithium recovery was that with the highest temperature and the lowest leaching agent concentration with no ultrasound, opposite to the initial hypothesis proposed in this work. Nevertheless, this recovery of lithium does not grow after a defined time frame, while the other do that. This means that the other two combinations of variables processes would require more time and resources to yield the same results, therefore costing more economically than the process at the highest temperature with low agent concentration and no ultrasound.
For cobalt and nickel, the recovered quantities where found to be lower than lithium, as is shown in Figure 2, even though this was the methods that yielded the most recovery for both metals , so one can assume that different methods should be used for the recovery of this elements.
Figure 1. The three best methods for the recovery of lithium showing the variables employed. 0.5 M concentration with no ultrasound and high temperature yielded the best result.
Figure 2. Results obtained for cobalt recovery at 1.5 M concentration and nickel at 0.5 M concentration.
This work presents an alternative choice to the known leaching processes based on sulfuric acid, even for metals that are not lithium and that are found in cellphone battery residues. The use of organic agents allows a less harmful impact to the environment and an easy way to handle residues.
The variation of the concentration of the leaching agent enabled to find the best combination of the involved variables, being 0.5 M for lithium and nickel recovery, and 1.5 M for cobalt. It was also seen that the highest temperature used (60˚C before the organic agent started to evaporate) yielded the highest concentration of the metals recovered.
For ultrasound, the best choice was 40 KHz for all the considered metals, however, in long time intervals, the curves for recovery present increasing growth but they never reach the highest possible efficiency. This indicates that ultrasound contributes to the breaking of the LiCoO2 compound in which the metals are found before leaching. Experiments indicate that the ideal conditions found for a high recovery efficiency, near 86%, are the highest temperature employed of 60˚C with no ultrasound. Future work must include additional experiments to find the conditions for an efficiency of metals recovery that be economically viable for pilot facilities, besides of testing different organic compounds and higher temperatures.