I have an alkaline solid waste which contains the following components: SiO2, Al2O3, and CaO according to the XRF analysis (as it gives the results as oxides). However, I know from the XRD that it also contains different compositions of these elements (such as CaCO3, Ca(OH)2, NaAlSi3O8).
My question is:
I want to recover the Al and Si containing fraction together, and get rid of the Ca containing fraction. Which is why, I consider to dissolve CaO in HCl and to form CaCl2. SiO2 will remain intact as it will not react with HCl. But Al2O3 will dissolve along with CaO in the form of AlCl3. Can I somehow arrange the pH and precipitate Al in the form of Al(OH)3 and separate Ca containing fraction in liquid form?
It's maybe not that easy to separate the components. Depending on the compounds and structure of your alkaline waste the Ca may not be easily dissolved. Especially some Ca silicates are hardly dissolved when they form protective silicate layers. You can find a lot of literature regarding the extraction efficiency of Ca with HCl, H2SO4 etc. from alkaline wastes.
Precipitation could be performed by adjusting the pH with NaOH at pH 4 - 7 for Al(OH)3 and pH > 12.5 for Ca(OH)2. Note that other components can co-precipitate.
Dear Mehmet,
This is a typical question for challenges related to cement, concrete and other construction materials, which comprise many Ca-Al-Si-O phases. What you need is an hydrothermal leaching and a complexing agent. Citric acid and amino acids are good ligands to separate Ca even from some REE. However, my proposal is to use Ethylenediaminetetraacetic acid (EDTA) as an very efficient chelating agent for Ca2+. At pH12 the stability maximum is reached, while the Al3+ and Si4+ EDTA complex is instable. Then you can separate the Ca-EDTA-complex, decompose it at pH 2-4, and finally precipiate Ca2+ as e.g. oxalate.
All the best for your research!
Thomas
The most optimized pH for Al(OH)3 precipitation alone could be 6, but consider a range for pH to design DOE. You can use NaOH to adjust the pH of leachate and to provide enough hydroxide to precipitate Al(OH)3. In my view, HCl could be a suitable solvent for your case, as CaCl2 is quite soluble and will not easily precipitate after leaching. In the case of using H2SO4 as the solvent, there is a high risk of precipitation of CaSO4 or gypsum, and Ca remain in the solid phase.
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