Washability and distribution of sulfur and trace elements for different size and density fractions of raw coals

Based on the hypothesis that the levels of sulfur and other toxic trace elements in a coal vary with particle size and density of the raw coal, this project studied their distribution in two raw coal samples to determine which size and/or density fractions of the unwashed coals should be targeted during processing to reduce the sulfur content.

The project's objectives were to:

• Quantify total sulfur, including organic and inorganic sulfur contents, for different size and density fractions of the selected raw coals.
• Quantify trace elements (As, B, Hg, Cd, F, Mo, Pb, Se & Zn) contents of different size and density fractions of raw coals.
• Identify which size and/or density fractions of raw coals should be specifically targeted during coal processing to reduce sulfur and trace element levels, and understand the impact on coal yield for high sulfur Australian coals.

The two raw coals used in this project, Coal 1 and Coal 2, had total sulfur contentsof  0.58% and 0.75%,  and ash contents of 37.3 and 46.9% respectively.  For Coal 1, the sulfur content for different size and density fractions was between 0.3% to 1.0% , and between 0.4% to 0.8% for Coal 2.

Size and density fractions of the two raw coals were prepared and analysed using both conventional and advanced techniques.  The main observations from the testing were:

• For particles from both coals, the total sulfur and organic sulfur contents decrease with the density, whereas the inorganic sulfur content shows the opposite trend.
• Product coal (<1.4 SG) from both raw coals contains mainly organic sulfur.  By contrast, the rocks (>2.0 SG) contain mainly inorganic sulfur.
• Sulfur in the product coal (<1.4 SG) is mainly contained in particles between 0.15 and 4 mm.
• For both coals, the high-density rocks (>2.0 SG) account for 30% of the total sulfur, and is mainly contained in particles larger than 16 mm.  Therefore, one third of total sulfur that is contained in the high-density fractions can be removed as rocks from the coal preparation plant.  However, some middlings (1.6-2.0 SG) may also be removed if sulfur needs to be further reduced.
• Overall, the content of trace elements in the raw coals increases with density except for molybdenum.  

The project outcomes are expected to aid in coal selection, preparation and blending for reducing sulfur release under increasingly stringent legislation in international markets, while maximising the yield of product coals from coal preparation plant.