Structural changes of lower rank coals by cation exchange

Abstract

Three subbituminous coals of different rank demineralized in HCl and HF aqueous solutions were treated with several metal salt aqueous solutions to produce corresponding cation exchanged coals. In this treatment, the demineralized coals were exchanged by cations (K +, Na +, Mg 2+, Ca 2+, Fe 2+, Fe 3+ and Al 3+), which are known to be the major elements in coal minerals. The relations between the content and the kind of exchanged cations were investigated. For lower rank coals, such as Adaro and Black Thunder coals, almost linear relations between the content of exchanged cations and the decreasing amounts of pyridine soluble yields of the cation exchanged coals based on the values in the respective demineralized coals were found. From the differences of the gradients, it was confirmed that the noncovalent associations such as hydrogen bonds between pyridine and coal molecules were proportionally reduced by cation exchange, that is, uni and divalent cations exchanged with one and two carboxyl groups bonded to coal molecules, respectively. Therefore, the contents of exchanged univalent cations were higher than those of divalent cations. Especially, the development of ionic crosslinking between one divalent cation and each carboxyl group in two different coal molecules was considered with increasing cation exchanges. Further, it was estimated that the weight of coal molecules with attached carboxyl groups in Black Thunder coal was 1.4 times larger than that in Adaro coal.