The alkali-extraction test as a reliable method of detecting oxidized metallurgical coal

Because oxidized coal is known to have an adverse effect on bulk-density control and coal handling in the coke plant, and on coke quality, U.S. Steel has developed the alkali-extraction test as a means to identify oxidized (weathered) coal in coal blends. In the test, coal is boiled in a caustic solution so that oxidized coal dissolves and darkens the solution. The solution is then tested for light transmittance, with the solution transmittance decreasing as the coal oxidation increases. A transmission value less than 80% indicates that the coal is too oxidized for metallurgical use. This rejection limit corresponds to 8 to 12% microscopically recognizable oxidized coal. Petrographic determinations of oxidized coal in four ranks of metallurgical coal show a linear relationship (0.964 correlation coefficient) with corresponding transmission values and a detection limit of three percent oxidized coal for the test. In addition, the oxygen content and infrared-band intensities of these coals also show a linear trend with transmission values. The study indicates that the alkali-extraction test is a reliable test for detecting oxidized coal in metallurgical coal blends and is superior to the Free-Swelling Index for setting mining limits in stripping operations. U.S. Steel is presently using the test to monitor captive and purchased coals for acceptance as metallurgical coal.