Pyrite flocculation

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. 

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. 

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. 

This principal environmental problem posed by coal-cleaning waste is that the pyrite and marcasite in the waste are oxidized to sulfuric acid in the presenee of air, water, Ferrobacillus ferrooxidans, and Thiobacillus ferrooxidans. The sulfuric acid is usually sufficiently concentrated to dissolve numerous metallic constituents and large quantities of iron from the pyrite in the leachate. Any ECT intended to prevent sulfuric acid formation must eliminate either the air, the water, or the oxidizable sulfur compounds in the waste, or inactive Ferrobacillus ferrooxidans and Thiobacillus ferrooxidans by maintaining alkaline conditions. Post-treatment of pile drainage comprises ECTs designed to neutralize the acid in the effluent and remove the metal ions by some sort of precipitation, adsorption, flocculation, or ion-exchange phenomenon. 

Several investigations were carried out to remove toxic heavy metal ions from waste water by biosorption. Microbial cells loaded with heavy metals were recovered by flotation, e.g. Streptomyces griseus and S clavuUgerus loaded with Pb [108] and Streptomyces pilosus loaded with Cd [109]. In these flotation processes the microbial cells were dead therefore, they are not considered here. The removal of pyritic sulfur from coal slurries such as coal/water mixtures by Thiobacillus ferrooxidans and recovery of this iron-oxidizing bacterium by flotation is a special technique in the presence of high concentrations of solid particles (see e.g. [110]). The flotation of colloid gas aphrons was used for the recovery of yeast in continuous operation [ 111 ] for the recovery of micro algae, and in the presence of flocculants in batch operation [112]. These special techniques are not discussed here. 

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