Sulfur removal data, pyritic

Table VIII. Pyritic Sulfur Removal Data"...

The data listed in Table V illustrate the effect of top mesh size on pyritic sulfur removal. The coal samples were prepared by the same comminution techniques, and consequently the size distribution of the samples should be similar for each case (8, 9, 10, 11, 12). The %,-inch mesh was as received coal, while the 14 X 0 and 100 X 0 mesh samples were obtained by grinding a representative sample in a ball mill until it... 

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. 

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. 

Table IV. Example of Sequential Processing of Coal for Removal of Pyritic and Organic Sulfur. (Current experimental data are given in the lined regions of the table.)...

Dissolved sulfide in this zone builds up because of its bacterial production. Its maximum concentration is less than the total sulfate reduced because a portion of the H2S reacts with iron and organic matter to form insoluble products. At any depth, the concentration gradient of H2S is kinetically controlled and reflects the balance between the rate of these removal processes, the rate of gain or loss by diffusion, and the rate of its formation by reduction. One of these processes, the production of H2S, must cease when all S04 has been consumed. The net result is a concentration maximum that falls in a range from 1 pM to >10 mM. The depth of maximum pore-water H2S commonly correlates closely with the depth of total S04 depletion. In most environments, H2S persists at measurable concentrations (i.e., greater than a few micromolar) in pore waters to depths of a few centimeters to several meters below the point at which S04 is removed. The essentially total removal of pore-water H2S is a reflection of the availability of excess iron over sulfide sulfur in most sediments (see below). Pyrite content may increase gradually within zone III, but the rate of this increase is most rapid at the top of this zone. Frequently, increases in pyrite cannot confidently be distinguished from scatter in the data within this zone. 

The effect of added hydrochloric acid concentration was studied in order to determine whether or not the acid had any effect on pyrite and ash removal, sulfate-to-sulfur ratio, final heat content, and possible chlorination of the coal. Coal has many basic ash constituents, so increased ash removal was expected, as well as some suppression of the sulfate-to-sulfur ratio because the reaction that results in sulfate formation also yields eight moles of hydrogen ion per mole of sulfate (common ion effect). Added acid was studied in the range of 0.0 to 1.2M (0.0, 0.1, 0.3, and 1.2M) hydrochloric acid in 0.9M ferric chloride. Duplicate runs were made at each concentration with all four coals for a total of 32 runs. The results showed no definite trends (except one-uide infra) even when the data were smoothed via computer regression analysis. Apparently the concentration range was not broad enough to have any substantial effect on the production of sulfate or to cause the removal of additional ash over that which is removed at the pH of IM ferric chloride ( pH 2). 

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