Minerals LIBS sorting

LIBS presently is mainly used for quantitative and semi-quantitative analyses. Much less attention has been paid to the potential of LIBS for qualitative analysis, which may be effectively used in radiometric sorting of minerals. In this sense we do not need to determine a detailed chemical composition of the minerals, but rather the aim is to find the specific lines, which enable us to identify the corresponding mineral. LIBS mineral sorting is in its early stages, but the prospects are very good. We developed the corresponding database of time-resolved LIBS under 1,064, 532, 355 and 266 nm laser excitations for the spectral range from 200 to 900 nm. 

It is evident that LIBS sorting may not be accomplished by integral irradiation, because it is based upon plasma analyses generated on relatively small area by focused laser beam. Nevertheless, if each pebble is composed of only one mineral, one laser pulse will be representative enough in order to determine the pebble composition. For uniformly embedded ores the situation is different and 2-5% of the pebble area has to be covered for the confident identification of useful mineral. Non-uniformly disseminated ores demand even more detailed scanning, which has to be determined individually for... 

About 90% of manganese ore is used in steel smelting. Although there are more than 300 manganiferous minerals, the common ore minerals are largely mixtures of manganese oxides and hydrated oxides. The usual field terms are psilomelane for a hard massive mixture of oxide minerals, pyrolusite for a soft black earthy mixture, and wad for impure, brown earthy oxides and hydrated oxides. LIBS sorting may be effective in this case (Fig. 8.11). 

Table 8.2. LIBS lines of different elements important for mineral sorting...

Luminescence and LIBS techniques may b e used simultaneously with the same basic equipment. This property may be effectively used in minerals sorting. Besides that, other methods such as Raman and SHG may be effective. 

Practically all world production of newly smelted aluminum was made from bauxite. The term bauxite is used for naturally occurring mixtures of aluminum monohydrate (boehmite or diaspore) and trihydrate gibbsite Al(OH)3, including impurities which are typically clay minerals, free silica, iron hydroxides and titania. The luminescence ofboehmite and diaspore and LIBS (Fig. 8.10) maybe used for the detection, sorting and commercial value evaluation of A1 minerals. 

These minerals have never been mentioned as potential targets for luminescent sorting because of the absence of natural emission even under laser excitation. LIBS may be an excellent opportunity, because all those elements have strong and characteristic lines. Figure 8.17 present examples of malachite (Cu), galena (Pb), and sphalerite (Zn) breakdown spectra. 

The possibilities of time-resolved laser based spectroscopies have been demonstrated, combining such techniques as luminescence, Raman, breakdown and second-harmonic generation. Radiometric sorting of minerals using LIBS was patented and apparatus for evaluation of phosphate rocks with elevated dolomite content has been developed and constructed. 

Luminescence sorting of fluorite is well known technique, which is based on strong blue luminescence of Eu. LIBS technique may be useful for fluorite ores sorting. Long lived emission of CaF molecule is a very effective distinctive feature of this mineral. 

---