Extraction of K, AI and Ti containing compounds from ash produced by low temperature combustion

Abstract

South African coal samples can contain up to 30% aluminium oxide which is constituted in the mullite and insoluble aluminium silicate mineral phases. This suggests that coal ash, produced in power plants or thermal processing of coal fines, has the potential to be a good source material for the recovery of aluminium. The aim of this investigation was to use modelling software to predict mineral transformation during thermal processing of the coal samples; after which the dissolution potential of aluminium from the amorphous material (metakaolinite (Al2O3.2SiO2)); and aluminium, potassium, and titanium in the amorphous aluminosilicate glass phases were determined. The starting material used in the recovery procedures was produced by low-temperature combustion (<1100°C) of South African coal samples. One of the coal samples was spiked with a 10 wt.% K2CO3 additive, which is used as a catalyst; to determine the influence this potassium compound had on the dissolution of aluminium, and subsequently potassium and titanium from the ash. FACTSAGE™ modelling software was used for the prediction runs; while three different recovery methods were used for the dissolution of the inorganic elements; i.e. H2SO4 leaching, (NH4)2SO4 sintering and NaOH leaching. FACTSAGE™ modelling software and accompanying databases were used to investigate the influence of operating conditions on the slagging behaviour of South African coal samples; along with the role that additives, such as potassium carbonate when added as a catalyst, have on the slagging behaviour. The results obtained through the prediction models indicated that the addition of potassium carbonate to the pulverized coal before thermal processing, lead to a decrease in melt formation temperature and a decrease in melt percentage. The extent of influence the potassium had on the coal behaviour during thermal processing, depended largely on the percentage of potassium present in the sample along with the composition of the coal. The basic components present in the coal will also influence the mineral transformation and slagging behaviour, due to their fluxing behaviour. Sulphuric acid leaching, with conditions similar to procedures used for the recovery of aluminium from clay sources, showed that coal ash prepared at 700°C yielded higher dissolution efficiencies of aluminium and potassium than the ash prepared at 1050°C. This is due to stable mineral phases present in the ash samples produced at higher temperatures. The addition of potassium carbonate to the coal sample resulted in higher dissolution efficiencies for aluminium from the ash. The highest dissolution efficiencies achieved were 87% Al and 89% K with the following experimental conditions: 700°C ash leached with a 6.12 M H2SO4 solution, using a solid to liquid ratio of 1:5 at a temperature of 80°C for 8 hours. The high dissolution efficiencies were due to the formation of acid-soluble amorphous phases, such as metakaolinite, K2CO3 melt, K2CO3, potassium-alum (KAl(SO4)2, and potassium aluminosilicate (gls). Subjecting the SA1 and SA2 blend ash samples prepared at 700°C to the ammonium sulphate sintering method, yielded maximum dissolution efficiency of 43% Al and 56% K for the SA1 ash sample (without K2CO3). These efficiencies were reached due to the formation of soluble ammonium aluminium sulphate and potassium sulphate. The addition of K2CO3 to the SA2 coal sample resulted in a lower Al dissolution efficiency due to the formation of insoluble potassium-alum. The dissolution of this mineral phase would require an added dissolution procedure. Alkaline leaching of the SA1 and SA2 blend ash samples yielded low dissolution efficiencies for Al and K when leached with the 1 M NaOH solution. Dissolution of the inorganic elements did not increase when an 8 M NaOH solution was used, however; the increase in alkaline solution concentration promoted the growth of sodalite (zeolite A) crystals within the ash. High K dissolution efficiencies of 59% and 89% were observed for the SA1 and SA2 blend ash respectively. The highest dissolution efficiencies were obtained by sequential leaching of either ash sample; with leaching conditions as follows: 4 hours leaching time, at a temperature of 80°C, using a 1:5 solid to liquid ratio. The overall conclusions made from this investigation is sulphuric acid leaching remains the most successful recovery method for aluminium. The addition of K2CO3 increased the dissolution efficiency of aluminium, even when low sulphuric acid concentration solutions were used. Ammonium sulphate sintering is another viable method for the recovery of aluminium from coal ash, but a different dissolution procedure is needed to solubilize the formed potassium-alum. Alkaline leaching of the ash samples did not yield good dissolution results, but the formation of zeolite crystals may be another viable utilization method for coal ash.