Pyrite dispersion

Other Sources of Selenium. In 1820 Leopold Gmelin prepared pure selenium from the fuming sulfuric acid of Graslitz [Kretzlitz] in Bohemia, and in the following year Buch and Wohler showed that this selenium came originally from the particles of iron pyrites dispersed in the alum shale from which the sulfuric acid had been prepared. 

Table 8 shows by example the problem of batch homogenization, where 1.4 kg pyrites and 4 kg salt must be uniformly dispersed throughout a 1000 kg batch, nearly one-third of which is 300 p.m (50 mesh) sand and nearly one-half of which is cuUet (1—2 cm) glass. 

This deposit is located in the north-east of Russia and belongs to a gold-arsenic type of low-sulphide formation (Abramson et al, 1980). It lies within a carbonaceous terrigenous rock of Triassic age and is associated with a dome-shaped uplift in a node of intersecting faults of various directions. The ore bodies consist of zones of silicification and kaolinization with veinlet-disseminated sulphide mineralisation. Gold is present in the form of finely-dispersed dissemination in arsenopyrite and pyrite. As an example. Figure 1 illustrates the distribution of Au and Mn in connection with commercial ore... 

A sulfidic halo is characterized mostly by elevated S, Au, As and Sb. It extends further than suggested by previous alteration studies, and is defined by the development of disseminated hydrothermal pyrite and, to a lesser extent, arsenopyrite. Gold deposits of the Costerfield stibnite domain (i.e., Fosterville and Costerfield) can be differentiated from Au-As orogenic deposits by a greater primary dispersion of anomalous As and higher threshold values for Sb, as well as by the presence of slightly elevated concentrations of Hg (>0.01 ppm). Other chalcophile elements at variably elevated levels within the sulfidic alteration halo include Mo, Se, Bi, Pb and Cu. 

Figure 1. Scanning electron photomicrographs of minerals from coals. The minerals were studied and photographed by a Cambridge Stereoscan microscope with an accessory energy-dispersive x-ray spectrometer at the Center for Electron Microscopy, University of Illinois. A. Pyrite framboids from the low-temperature ash of a sample from the DeKoven Coal Member. B. Pyrite cast of plant cells from the low-temperature ash of a sample from the Colchester (No. 2) Coal Member. C. Kaolinite (left) and sphalerite (right) in minerals from a cleat (vertical fracture), Herrin (No. 6) Coal Member. D. Calcite from a cleat in the Herrin (No. 6) Coal Member. E. Kaolinite books from a cleat in the Herrin (No. 6) Coal Member. F. Galena small crystals in the low-temperature ash of a sample from the DeKoven Coal Member.

The principal use of forms of sulfur data is in connection with the cleaning of coal. Within certain limits, pyrite sulfur can be removed from coal by gravity separation methods, whereas organic sulfur cannot. Pyrite sulfur content can therefore be used to predict how much sulfur can be removed from the coal and to evaluate cleaning processes. If the pyrite sulfur occurs in layers, it can usually be removed efficiently. If it occurs as fine crystals dispersed throughout the coal, its removal is very difficult. 

Mineralized coal impure coal that is heavily impregnated with mineral matter, either dispersed or discretely localized along cleat joints or other fissures pyrite and calcareous minerals are the most common (ASTM D-2796). 

Separation of Ultrafine Pyrite from High Sulfur Coals by Selective Dispersion and Flocculation... 

A novel technique for separating ultrafine pyrite particles (minus 1 0 micrometers) from coal fines has been conceptually developed and tested. The technique involves the use of a selective polymeric dispersant for pyrite, while flocculating coal particles with a polymeric flocculant. The suspended pyrite can then be removed from the flocculated coal fines which settle preferentially by gravity. 

The key to this separation was the design and preparation of the selective dispersant for pyrite (PAAX). 

One of the promising new technologies for separation of very fine particles is selective flocculation. The selective flocculation process has been used effectively to separate very finely disseminated minerals from mixed ore suspensions (5.). The process is based on the preferential adsorption of an organic flocculant on the wanted minerals, thereby flocculating them, while leaving the remainder of the suspension particles dispersed. The dispersion of certain components in the suspension such as pyrite can be enhanced by using more selective or powerful dispersants. Methods for achieving selective flocculation and dispersion have been recently described by Attia (6j. 

At the end of the settling period, the suspended solids were decanted, and the settled solids were recovered. Each fraction was placed in an evaporating dish, oven dried and weighed. Selective flocculation of coal mixtures with pyrite was made on suspensions containing equal proportions of coal and pyrite, using 200 mg/l PAAX dispersant at pH 10. The flocculation procedure was the same as described above, except that the products were qualitatively analyzed by visual inspection of both fractions. The coal samples used in these experiments were anthracite coal, supplied by Wilkes-Barre, Pennsylvania, and the pyrite used was pure crystals from Wards Natural Sciences, Inc., Rochester, N.Y. 

The flocculation results on the individual mineral suspensions are shown in Figure 2 (A B). These graphs show the effect of polyacrylic acid dispersant before (PAA) Figure 2A, and after xanthation (PAAX) Figure 2B, on the flocculation-dispersion behavior of individual suspensions of coal and pyrite with Purifloc-A22 flocculant. 

From Figure 2(A), it appeared that PAA inhibited or restrained the flocculation action of Purifloc-A22 on both coal and pyrite suspensions at PAA concentrations of 100 mg/l and above. The dispersive action of PAA in this case was therefore unselective. However, the PAAX crude reaction product in Figure 2(B) only dispersed the pyrite suspension to the same level as PAA, while the coal suspension was totally flocculated even at high PAAX concentrations. 

The polyxanthate dispersant, rather than improving the dispersion of pyrite, simply did not adsorb on the coal particles, thereby creating a selective dispersion action for the pyrite. These observations in Figure 2(B) were repeated and noted several times, even with purified PAAX solutions. Selective dispersion of pyrite or... 

Figure 2. Effect of (A) Polyacrylic Acid (PAA) and (B) Xanthated Polyacrylic Acid (PAAX) Dispersants on the Flocculation of Anthracite Coal and Pyrite Suspensions with Purifloc-A22 (2 mg/1) at pH 10. Reproduced with permission from Ref. 9, Copyright 1985, Elsevier.

In order to ascertain that the selective dispersion effect of PAAX was truly due to the modified polymer itself and not to the associated poly-sulfides in the crude reaction, the flocculation testing was repeated with the purified PAAX solution. By using 300 mg/l of the purified PAAX solution, about 96 percent of the coal suspension flocculated in 5 minutes, while the pyrite suspension remained stable. These tests confirmed that the selective dispersion action was due to the PAAX (polyxanthate polymer) itself. 

Effect of Pyrite Particle Size on Dispersion. It was suspected that a lot of the apparently non-dispersed pyrite particles shown in Figure 2 was due to the settling of hoarse particles between 10 and 37 micrometers. Pyrite has a specific gravity of about 5 0, while that of coal is around 1.2-1.3 Therefore, a pyrite suspension having only particle size below 10 micrometers was prepared and tested. The results, which are also shown in Figure 2(b), showed that the minus 10 micrometer pyrite suspension remained very stable, with only 10 - 20% weight of the particles settled or flocculated. From these observations, it is believed that the selective dispersion of pyrite will be more effective for the smaller particle sizes. 

The lower rejection ratio of 16 was accompanied by high selectivity in pyritic sulfur dispersion. This was due to the higher (10 mg/l) flocculant concentration which resulted in higher coal yield (93.1 wt) in the flocculated fraction. On the other extreme, when higher dispersant concentration (500 mg/l) was used with lower flocculant concentration (2 mg/l), much less coal was flocculated (77 wt) and more sulfur was apparently rejected (39 ). The intermediate conditions of 300 mg/l PAAX dispersant and 2 mg/l flocculant produced correspondingly intermediate results. 

Fig. 6.13. Results of a band-structure calculation on pyrite showing dispersion of energy bands along some principal symmetry directions (after Lauer et al., 1984 reproduced with the publisher s permission).

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