Acidified subcritical water extraction of major ash-forming elements from coal

Acidified subcritical water is proposed for the extraction of major-ash forming elements (namely, aluminium, iron, calcium, magnesium, sodium and potassium) from coal prior to determination by atomic spectrometry. Coal samples (0.2 g) were subjected to extraction with water (containing with 4% (v/v) HNO3). An in-depth study of the variables affecting the leaching step as well as those concerning the chemical nature of the extractant was performed. A critical comparison between the use of a mild extraction mode (using an extractor made of stainless steel) and a drastic extraction mode (using an extractor made of hastelloid) was performed in terms of cost, analysis time and quality of the results. The kinetics of the leaching process for the two modes was also studied. Both alternatives avoid pretreatment steps and handling of organic solvents. The method was validated by using a black coal certified reference material (NIST-SRM 1632b). The excellent precision of the method (RSDs between 2.6% and 11.6% and between 1.0% and 6.2% for the mild and drastic mode, respectively), together with its safety and rapidity make it a good alternative for the determination of these analytes in coals. (C) 1999 Elsevier Science B.V. All rights reserved.