Sulphur Removal From Sulcis Coal By Sequential Leaching With Koh Followed By H2o2
E. Fois (1), A. Pistis (1), F. Melis (1), G. Mura (2) and A. Lallai (2) (1) Carbosulcis S.p.A., Departement Res. &Dev. Miniera Monte Sinni, 09010 Gonnesa (CI), Italy – e-mail: carbo@tiscali.it (2) Associated Professor Dipartimento di Ingegneria Chimica e Materiali, Università di Cagliari,Piazza d’Armi, 09123 Cagliari (CA), Italy The present work shows the results of the experimental tests conducted on Sulcis coal samples by sequential leaching of potassium hydroxide followed by hydrogen peroxide. The tests carried out verify the technical efficiency of the process for desulphurization of that coal. Sulcis coal is a sub-bituminous coal from the Monte Sinni mine in the South West part of Sardinia island, Italy. The ash content of this coal is about 15 wt % as received, whereas the sulphur content is 7-6 wt% (inorganic sulphur is present at about 3%). The tests carried out verified the technical efficiency of the process for desulphurization of Sulcis coal. Some leaching tests allowed us to obtain a mineral fraction of about 12% and a sulphur content of about 4%.

1. Introduction
Coal is the most abundant fossil fuel on earth. In particular, semi-anthracite and anthracite are fossil coal in the real sense of the word and are the ones most exploited on a large scale, being the richest in carbon content and hence provided with the highest calorific value. However, in recent decades, one of the disadvantages related to the use of coal for energy production is the emission in the environment of sulphur oxides and hydrogen sulphide. Sulphur in coal occurs in the forms of organic and inorganic substances. The inorganic sulphur is present mainly in two forms, disulfides (pyrite and marcasite) and sulphate (mainly calcium, iron and barium). The organic form, which is bound directly to the organic coal matrix, generally occurs in the forms of thiols, sulphides, disulfides, thiophenes and cyclic sulphides. Various methods of coal desulphurization are currently in use: physical, chemical, and biological. Since the organic sulphur is distributed through the coal matrix as an integral part of the molecular structure, its removal requires chemical treatment. However, pyritic sulphur might be removed by physical cleaning processes to different extents. Thus, there are two main coal desulphurization processes prior to its utilization: chemical processes directed usually to oxidation or reduction of sulphur; and physical processes (Kawatra and Eisele, 2001).

Many laboratory and industrial processes have been developed to partially or largely reduce the presence of sulphur through chemical processing and a large number of papers is available on literature about these techniques. Experimental tests involve mainly strong mineral acids (such as Hydrochloric, Nitric, Hydrofluoric, and Sulphuric Acid) and oxidizing agents (such as metallic salts, chlorine gas, peroxides, sodium hypochlorite, etc.). Sulcis coal is a sub-bituminous coal from the Monte Sinni mine in the South West part of Sardinia island, Italy. The ash content of this coal is about 15 wt % as received, whereas the sulphur content is 7-6 wt% (inorganic sulphur is present at about 3%). Table 1 shows the composition of the mineral matter as fraction of oxides while Figure 1 shows the XRD pattern of ash obtained by LTA and also by conventional ashing at 850°C. The Sulcis coal, classified as sub-bituminous, has a low heating value (as received) of 20.14 – 21.76 MJ/kg. Since most of the pyritic sulphur is finely disseminated in crystals of micron or sub-micron size (Carbini et al., 1990), the only remaining possibility of removing sulphur from this coal prior to its utilization is confined to chemical methods.

Figure 1 XRD spectrum of Sulcis coal ash obtained by low and high temperature methods

Table1 Composition of the mineral matter as fraction of oxides (Analysis XRF) Parameter SiO2 Fe2O3 MgO