Removal of pyritic sulfur from coal

Chemical Removal of Pyritic Sulfur from Coal... 

A new approach for the chemical removal of pyritic sulfur from coal is described. The process is based on the discovery that aqueous ferric salts selectively oxidize the pyritic sulfur in coal to chemical forms which can be removed by vaporiza-tion, steam, or solvent extraction. Data for removal of the pyritic sulfur from four major coals (Lower Kittanning, Illu nois No. 5, Herrin No. 6 and Pittsburgh) are presented together with a discussion of the process chemistry. The effect of variables, such as coal particle size, acid and iron concern tration, reaction time, and temperature are discussed. The results show that near complete removal of pyritic sulfur can be obtained under mild conditions, resulting in a reduction of the total sulfur content of the coals from 40 to 80%, depending on the original pyritic sulfur content. 

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. 

Joshi, J.B., Y.T. Shah, R.S. Albal, H.J. Ritz and W.D. Riche. "Effect of pH on the Removal of Pyrite Sulfur from Coal by Oxyde-sulfurization." a paper submitted to Ind. Eng. Chem. Proc. Des. 

Richey. Effect of pH on the Removal of Pyritic Sulfur from Coal by Oxydesulfurization. a paper submitted to I EC Process Design and Dev. (1981). 

Growing concern over environmental effects of acid rain has resulted in increased Interest in development of pre-combustion removal of sulfur from coal. Physical coal cleaning processes are effective for pyritic sulfur removal but do little to reduce the organic sulfur content of coal This paper reports the removal of organic sulfur from coal, employing ethyl or methyl alcohols as the solvent/ reactant. The process is based on the observation that, under supercritical conditions, reactions occur that selectively remove organic sulfur from the coal matrix. 

Colmer and Hinkle (14) identified T. ferrooxidans in acidic mine waters. Subsequent studies by Silverman et al. (15,16) confirmed that T. ferrooxidans could be utilized to oxidize FeSo in coal in 3 to 4 days, and the rate of oxidative dissolution was a function of the particle size and rank of the coal. Dugan and Apel (4,5) showed that a mixed culture of T. ferrooxidans and T. thiooxidans was most effective at a pH of 2 to 2.5 when the nutrient was enriched with NH " -. They reported 97% removal of pyritic sulfur from a coal sample with 3.1 weight percent sulfur. Norris and Kelly (17) reported that other acidophilic bacteria, Leptospirillum ferrooxidans in mixed cultures with T. thiooxidans, was effective for FeS2 removal. ... 

A number of processes are being used to remove sulfur and sulfur oxides from fuel before combustion and from stack gas after combustion. Most of these efforts concentrate on coal, since it is the major source of sulfur oxides pollution. Physical separation techniques can be used to remove discrete particles of pyritic sulfur from coal. Chemical methods can also be employed for removal of sulfur from coal. 

The goal of beneficiation is to remove as much sulfur from a fuel as possible before it is ever burned. When burned, fuel with lower sulfur content will produce less sulfur dioxide. Beneficiation is usually accomplished by a physical process that separates one form of sulfur, pyritic sulfur, from coal. Pyritic sulfur consists of sulfur minerals (primarily sulfides) that are not chemically bonded to coal in any way. The name is taken from the most common form of mineral sulfur usually found in coal, pyrite, or iron sulfide (FeS2). 

Under appropriate conditions, over 90% of the pyritic sulfur from coals can be removed by the mesophilic sulfur-oxidizing autotrophic bacteria Thiobacillus ferroxidans and T. thiooxidans, but these bacteria were incapable of removing organic sulfur (Khalid et al., 1989,1990a,b Bhattacharyya et al., 1990 Huffman et al., 1990). However, the thermophilic archaebacterium Sulfolobus brier-layi was able to remove over 95% of the pyritic sulfur and over 30% of the organic sulfur from the untreated coal when the cells of S. brierleyi were acclimatized. The aerobic biosolubilization of low-rank coal to polar water-soluble products has been demonstrated (Scott et al., 1986 Cohen et al., 1987 Pyne et al., 1987 Wilson et al., 1987). 

At present, pyritic sulfur from coal can be easily removed by oxidation. The removal of organic sulfur generally requires severe operating conditions which also cause undesirable carbon loss. Future work must consider the removal of organic sulfur by the use of homogeneous catalysts under mild conditions. The processing of a low pH slurry could be an important factor in the commercialization of an oxydesulfurization process. 

The developed process aimed for the removal of iron and sulfur from coal. While several patents for microbial removal of pyrite from coal exist, this one is unique because of the microbial mixture developed by the group for this purpose, which is partially... 

Rudimentary investigations of microbial desulfurization have received little attention in the literature at this time (2). One successful example of desulfurization is the removal of pyrite from coal by Thiobacillus sp. and Ferrobaccus sp. (3). While studies of the complex hydrocarbon-sulfur systems are of great value, being closer to in situ reality, investigation of a defined system should form the foundation of these more detailed studies. 

The full potential for removing pyritic sulfur from various coals by physical coal cleaning is significant but difficult to achieve. However, SO2 control by precombustion removal of pyrite could be an important S02-emissions reduction strategy. The cleaned coal produced could be used in coal-fired utilities, constructed both pre-and post-NSPS, as well as in industrial boilers. To realize the potential for coal cleaning in actual practice, however, new techniques must be demonstrated in the laboratory and then at the "proof-of-concept" scale (approximately one ton of coal per hour). These new coal beneficiation techniques could be advanced physical-coal-cleaning (PCC) processes, or they could employ microbial desulfurization or chemical desulfurization to remove organic sulfur. These latter processes could be used by themselves or in concert with PCC processes. 

It is believed, therefore, that the separation-removal of pyrite from coal prior to its combustion would greatly reduce the sulfur dioxide emission and render many coal deposits within the new source performance standards. 

Ferric sulfate has several advantages over ferric chloride in a process, so a test matrix was performed (summarized in Table VI) to compare the abilities of ferric sulfate and -chloride to remove pyritic sulfur from all four coals. Slightly less sulfur was removed by 0.4N ferric sulfate than was indicated with 0.4N ferric chloride. However, 0.9N ferric sulfate removed an equal or greater amount of sulfur than 0.9N ferric chloride. 

Attempts to increase pyrite removal by increasing the reaction time met with limited success under our standard conditions because reaction of the ferric ion with the coal matrix depleted the ferric ion that was needed for extraction of the pyrite. Thus, for example, increasing the coal reaction time from 2 to 12 hrs only increased pyritic sulfur removal from 60 to 80% for Pittsburgh coal. Similar results were obtained for the other three coals. The only alternatives were to increase the amount of leach solution or to use a continuous or semi-continuous (multiple-batch) reactor. A multiple-batch mode was chosen because it was a simple laboratory procedure and at the same time it could approximate conditions encountered in a commercial plant. A 1-hr-per-batch leach time was used because our 2 hr results indicated that in the early stages of removal the rate begins to decrease after 1 hr, and six leaches (or batches) per run were used to assure that any pyrite that could be removed in a reasonable amount of time was removed. The progress of removal was monitored by analyzing the sulfate content in each spent leach solution elemental sulfur was not removed until all the leaches were completed. Table VII shows pyrite extraction as a function of successive leaches as followed by sulfate analysis of the leach solution. Note that the major portion of pyritic sulfur is removed in the first two leaches or 2 hrs, followed by lesser amounts in... 

Organic snlfnr comprises 50%-60% of the total snlfur present in coal it is an integral part of the coal strnctnre and cannot be removed by mechanical means (Chapter 6). Pyritic sulfur accounts for most of the remaining sulfur in coal. Gravity separation techniques can readily remove pyritic snlfur from coal if the pyrite particles in the coal are fairly large. The coal industry has used these techniques for many years. Many American coals permit the removal of about half of the pyritic sulfur in this way. The pyrite in some coals, however, is too fine to permit separation by these methods. 

A fascinating little company in Canada was producing pyrites to recover sulfur from coal gas. In 1882, the Vesey Chemical Company of Montreal commenced burning spent oxide from the city s gasworks into sulfur dioxide. The spent oxide was iron ore that had been used to remove hydrogen sulfide from coal gas. The plant was purchased by Nichols Chemical in 1902 and shut down. 

The primary source of anthropogenic sulfur dioxide is coal, from which sulfur must be removed at great expense to keep sulfur dioxide emissions at acceptable levels. Approximately half of the sulfur in coal is in some form of pyrite, FeS2, and the other half is organic sulfur. The production of sulfur dioxide by the combustion of pyrite is given by the following reaction ... 

In an extensive study by Read et al. [93], 10 anionic surfactants were evaluated for their ability to remove pyritic sulfur and ash from ultrafine Illinois no. 5 coal by flotation processes. The authors observed that of the commercially available surfactants, sodium dodecyl sulfate was the most effective on either a weight or a molar basis, followed by a linear AOS (average molweight 272) and alkylpolyethoxylated sulfonates. Of the noncommercial surfactants tested, -(E -b-dodecene-b-suIfonate (f0) was the most effective and better than any commercial surfactant on a dosage/recovery basis. 

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