Gravimelt process

J. L. Anastasi and co-workers, "Molten-Caustic-Leaching (Gravimelt Process) Integrated Test Circuit Operation Results," Report to the Gravimelt Process Advisory Board, Summer 1989. 

Effect of Gravimelt Process on Mineral Matter in Coal. The samples of Pittsburgh No. 8 coal obtained from TRW, representing raw coal, the intermediate product and the final cleaned coal, were analyzed by ASTM and other procedures. The results are presented in Table V. In the final product, the total and organic sulfur were reduced by 70-80% from the levels observed in the raw coal, and the pyritic sulfur was reduced by about 99%, while levels of sulfate sulfur were not significantly affected. The organic sulfur concentrations... 

In addition, the TRW samples were analyzed by the SEM-AIA technique described previously (13). The SEM-AIA data on the mineral phase identification and distribution between the size fractions are presented in Table VI for all three samples of raw and treated coal. The SEM-AIA data show the nearly complete removal of many minerals and a reduction of more than 90X in the overall content of the coal as a result of treatment by the TRW Gravimelt Process. No major changes in particle size distribution were observed, although the pyrite distribution shifted somewhat towards the coarse fraction after processing. 

Coal treated at the Ames Laboratory with molten caustic by a procedure similar to that of the Gravimelt Process resulted generally in 80-90% reductions in ash and 70-80% reductions in total sulfur. The recoveries of coal on a moisture- and ash-free basis were 80-90%. 

TRW Energy Development Group, "Gravimelt Process Development " Final Report, D0E/PC/42295-T7 (DE84013743), Redondo Beach, CA, June 1983. 

In addition to these samples, several coal samples were provided by TRW Systems, Inc., from the San Juan Capistrano test plant in California. The samples included Illinois No. 6 and Pittsburgh No. 8 coals before and after treatment by the Gravimelt Process. The raw Illinois and Pittsburgh coals were -14 mesh and -6 mesh, respectively. 

Results of chemical analyses on coals treated by the Gravimelt Process at TRW are shown in Table IV. Ash contents were decreased to 1% or less, and levels of As, Be, Cd, Hg, Pb, Se, Sr, and Zn were reduced by 75-95%. It is interesting to note that most of these elements are commonly associated with pyrlte. Thus, substantial reductions in levels of these elements can be anticipated when most of the pyrlte is removed (as indicated for both coal samples). In addition, the levels of Ba, Ge, Mn, and Rb were also reduced however, the reduction levels for these elements are not as prominent nor as clear-cut. 

FIGURE 26 Molten-Caustic-Leaching (Gravimelt Process) flow schematic. 

A schematic of the Gravimelt Process is shown in Fig. 26. Feed coal is premixed with anhydrous sodimn hydroxide or mixed sodium and potassium hydroxides, and then fed to a rotary kiln reactor where the mixture is heated to reaction temperatures of 325 15°C, causing the caustic to (1) melt and become sorbed in the coal matrix, (2) react with the coal sulfur and mineral matter, and (3) dissolve the reaction products containing sulfur and inorganic components. The kiln reactor can be seen in operation in Figiue 27. 

Chemical cleaning processes utilize alkali or acid to leach the impurities present in the coal. TRW s Gravimelt process, which has been tested on a pilot scale, utilizes molten caustic to leach mineral matter. The major drawback of a chemical cleaning process is the economics. 

The pyritic sulfur in coal can undergo reaction with sulfate solutions to release elemental sulfur (see Sulfurremoval and recovery). Processes to reduce the sulfur content of coal have been sought (75). The reaction of coal and sulfuric acid has been used to produce cation exchangers, but it was not very efficient and is no longer employed. Efforts have turned to the use of hot concentrated alkaH in a process called Gravimelt. 

Gravimelt A coal cleaning process in which coal is heated with molten sodium hydroxide and then washed with acid. The process removes 90 percent of the sulfur and 95 percent of the ash. It was planned to be piloted in 1987. 

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