Journal of Sustainable Metallurgy

, Volume 3, Issue 4, pp 782–792 | Cite as

Purification of Coal Fly Ash Leach Liquor for Alumina Recovery Using an Integrated Precipitation and Solvent Extraction Process

  • M. Rampou
  • S. Ndlovu
  • A. ShemiEmail author
Research Article


The purification of coal fly ash (CFA) leach liquor has been carried out using an integrated solvent extraction and precipitation technique. The integrated technique was employed to primarily remove two dominant impurities, Ti and Fe, from CFA leach liquor. The solvent extraction experiments were carried out at varying factor values of contact time, aqueous-to-organic ratio, and extractant concentration. Preliminary results showed no loading of Fe2+ onto the organic phase, partial loading of Fe3+, and a high loading of Ti4+. Unable to adequately load onto the organic phase due to the high CFA leach liquor acidity at pH 0.1, Fe3+ required reduction to Fe2+ in order to bypass the solvent extraction stage. To achieve this, an integrated solvent extraction and precipitation technique was formulated and adopted. The technique involved iron reduction from Fe3+ to Fe2+ as the first treatment of CFA leach liquor, followed by solvent extraction of Ti4+ and subsequent recovery of titanium as TiO2, crystallization of Al3+ and the precipitation of Fe2+, in the form of Fe(OH)2, as the last treatment. Alumina was recovered as a precipitate product in the form of (NH4) Al(SO4)2·12H2O, and the calcined alumina product contained 99.91% Al2O3. Primene JMT was used in small amounts of ~10% in kerosene for short contact times of ~15 min using an A/O ratio of 1:1 in order to achieve high extraction efficiencies during the solvent extraction step. The results presented in this paper identify the integrated precipitation and solvent extraction route as an effective option for CFA leach liquor purification and alumina recovery.


Coal fly ash Alumina Solvent extraction Precipitation 



The authors thank Kendal Power Plant, a division of Eskom, for the CFA used in this study, and AngloGold Ashanti for use of their facilities for sample analysis. The authors thank the National Research Foundation and Department of Science and Technology (South Africa) for their financial contribution to the research.


  1. 1.
    Sibanda V, Ndlovu S, Dombo G, Shemi A, Rampou M (2016) Towards the utilization of fly ash as a feedstock for smelter grade alumina production: a review of the developments. J Sustain Metall 2:167–184CrossRefGoogle Scholar
  2. 2.
    Shemi A, Mpana RN, Ndlovu S, van Dyk LD, Sibanda V, Seepe L (2012) Alternative techniques for extracting alumina from coal fly ash. Miner Eng 34:30–37CrossRefGoogle Scholar
  3. 3.
    Shemi A, Ndlovu S, Sibanda V, van Dyk LD (2014) Extraction of alumina from coal fly ash: identification and optimization of influential factors using statistical design of experiment. Int J Miner Process 127:10–15CrossRefGoogle Scholar
  4. 4.
    Rahaman MA, Gafur MA, Kurny ASW (2013) Kinetics of recovery of alumina from coal fly ash through fusion with sodium hydroxide. Am J Mater Eng Technol 1:54–58Google Scholar
  5. 5.
    Shemi A, Ndlovu S, Sibanda V, van Dyk LD (2015) Extraction of alumina from coal fly ash using an acid leach-sinter-acid leach technique. Hydrometallurgy 157:348–355CrossRefGoogle Scholar
  6. 6.
    Sole KC (1999) Recovery of titanium from leach liquors of titaniferous magnetites by solvent extraction part 1. Review of literature and aqueous thermodynamics. Hydrometallurgy 51:239–253CrossRefGoogle Scholar
  7. 7.
    Rushwaya MJ, Ndlovu S (2017) Purification of coal fly ash leach liquor by solvent extraction: identification of influential factors using design of experiments. Int J Miner Process 164:11–20CrossRefGoogle Scholar
  8. 8.
    Matjie RH, Bunt JR, Van Heerden JHP (2005) Extraction of alumina from coal fly ash generated from a selected low rank bituminous South African coal. Miner Eng 18:299–310CrossRefGoogle Scholar
  9. 9.
    Somiya S (2003) Handbook of advanced ceramics: materials, applications, processing and properties. Academic Press, New YorkGoogle Scholar
  10. 10.
    Monhemius AJ (1977) Precipitation diagrams for metal hydroxides, sulfides, arsenates and phosphates. Trans Inst Min Metall 86:202–206Google Scholar
  11. 11.
    Gani R, Jimenez-Gonzalez C, Kate A, Crafts PA, Powell MJ, Powell L, Atherton JH, Cordiner JL (2006) A modern approach to solvent selection. Chem Eng 113:30–43Google Scholar
  12. 12.
    Seeley FG, McDowell WJ, Felker LK, Kelmers AD, Egan BZ (1981) Determination of extraction equilibria for several metals in the development of a process designed to recover aluminium and other metals from coal combustion ash. Hydrometallurgy 6:277–290CrossRefGoogle Scholar
  13. 13.
    Chou KS, Lin CC (1986) Extracting iron from aluminium sulphate solution. Hydrometallurgy 15:391–397CrossRefGoogle Scholar
  14. 14.
    Li LS, Wu YS, Li YY, Zhai YC (2011) Extraction of alumina from coal fly ash with sulfuric acid leaching method. Chin J Process Eng 11:255–258Google Scholar
  15. 15.
    Zhu P, Hong D, Wu J, Qian G (2011) Kinetics of forward extraction of ti(iv) from sulphuric acid medium by P507 in kerosene using the single drop technique. Rare Met 30:1–7CrossRefGoogle Scholar
  16. 16.
    Kasey JB (1971) Process for the selective separation of ferric sulphate from copper in a sulphuric acid leach solution. US Patent US3586498 AGoogle Scholar
  17. 17.
    Vainshtein BK (1994) Fundamentals of crystals: symmetry, and methods of structural crystallography. Springer, BerlinGoogle Scholar
  18. 18.
    Muwanguzi AJB, Karasev AV, Byaruhanga JK, Jönsson PG (2012) Characterization of chemical composition and microstructure of natural iron ore from muko deposits. ISRN Mater Sci 2012:174803Google Scholar
  19. 19.
    Australian Mines Atlas (2016) Australian Atlas of mineral resources, mines and processing centres. Accessed 6 Nov 2016
  20. 20.
    ASTM (1988) Standard specification for titanium dioxide pigments. In: Storer RA, Cornillit JL, Savini DF et al (eds) 1988 annual book of ASTM standards: paint–pigments, resins, and polymers. American Society for Testing and Materials, Philadelphia, pp 100–101Google Scholar

Copyright information

© The Minerals, Metals & Materials Society 2017

Authors and Affiliations

  1. 1.School of Chemical and Metallurgical EngineeringUniversity of the WitwatersrandJohannesburgSouth Africa
  2. 2.School of Chemical and Metallurgical Engineering, DST/NRF SARChI: Hydrometallurgy and Sustainable DevelopmentUniversity of the WitwatersrandJohannesburgSouth Africa

Personalised recommendations