Coal Preparation » Fine Coal
Dewatering of fines is problematic due to the high surface area and the varied nature of the feeds to the various centrifugal dewatering machines. In particular, tailings containing different clay types and amounts can reduce the capacity and increase the cake moisture of the dewatering machines. Producers and plant designers currently have no published simple laboratory tests that can estimate the final cake moistures from centrifuges processing coal in size below 2.0 mm. Several researchers have developed models that will provide this estimate, but they rely on a good knowledge of the material being fed to the centrifuge and centrifuge operating conditions.
This project describes laboratory tests to simulate the operations of three centrifuges, a fine coal dewatering laboratory test validated to provide a very good estimate of the final product moisture from a fine coal (scroll) centrifuge, a screen bowl laboratory test and a solid bowl laboratory test method.
The fine coal centrifuge test provides a moisture estimate for a particular coal at an operating g force. The test was validated against fine coal centrifuge plant data, and the mean average percentage error (MAPE) between the estimate and the plant data was 0.78%. A simple model that requires only the input of the Non-Centrifugable Moisture (NCM), which is determined using this test, has been developed and validated against 14 data sets.
The screen bowl centrifuge laboratory test provides an estimate of the expected cake moisture from a screen bowl centrifuge as well as the NCM for that coal at the operating g force. The results have been validated against three different coals, and the results had a MAPE of 1.97%. A model was also developed for a data set of 33 samples that showed that the g force, time in the centrifuge, and NCM were the most influential factors that affected the final cake moistures. The feed solids contents were held constant for all of these tests; therefore, the feed solids content effects could not be estimated.
The third test under development is for a solid bowl centrifuge laboratory test that is currently being developed for both fine coal and tailings dewatering. Some excellent initial results have been obtained from this laboratory test; however, when compared to the plant solid bowl centrifuges, the test gave 1.6% to 6.5%% higher moisture readings with a MAPE for three tests performed by different operators of 3.96%. These higher readings are believed to be due to the almost complete capture of solids in the test apparatus. Operating screen bowl centrifuges can have centrate solids ranging from 5% to 10%, while the % centrate solids in the laboratory test had solids contents less than 1%.
Tailings samples were also processed using this test procedure; however, the estimated cake moistures could only be compared with belt press filter results provided by the plant, which were reported as a range of values. The laboratory test results for the four tailings samples were at the lower end of the range of values in all cases. More work is required on the apparatus used for the solid bowl centrifuge laboratory test and in characterising the feeds, particularly in tailings dewatering where clay types and amounts can have a significant effect on the final product moistures.