Structural changes of lower rank coals by cation exchange

Three subbituminous coals of different rank demineralized in HCl and HF aqu eous solutions were treated with several metal salt aqueous solutions to pr oduce corresponding cation exchanged coals. in this treatment, the deminera lized coals were exchanged by cations (K+, Na+ Mg2+, Ca2+, Fe2+, Fe3+ and A l3+), which are known to be the major elements in coal minerals. The relati ons between the content and the kind of exchanged cations were investigated . For lower rank coals, such as Adaro and Black Thunder coals, almost linea r relations between the content of exchanged cations and the decreasing amo unts of pyridine soluble yields of the cation exchanged coals based on the values in the respective demineralized coals were found. From the differenc es of the gradients, it was confirmed that the noncovalent associations suc h as hydrogen bonds between pyridine and coal molecules were proportionally reduced by cation exchange, that is, uni and divalent cations exchanged wi th one and two carboxyl groups bonded to coal molecules, respectively. Ther efore, the contents of exchanged univalent cations were higher than those o f divalent cations. Especially, the development of ionic crosslinking betwe en one divalent cation and each carboxyl group in two different coal molecu les was considered with increasing cation exchanges. Further, it was estima ted that the weight of coal molecules with attached carboxyl groups in Blac k Thunder coal was 1.4 times larger than that in Adaro coal. (C) 1999 Elsev ier Science Ltd. All rights reserved.