Pyrite removal

We studied the effect of acid concentration, coal particle size, ferrous and sulfate ion concentrations, and reaction time on pyrite removal. These parameters were studied under conditions see Experimental) that give 40-70% pyritic sulfur removal, rather than 85-90%, so that the effects of parameter variations are clear and not so small as to be masked by experimental error. In addition, studies were performed to demonstrate... 

Table III) results in only a 4 5% loss in the heating value. Thus for a 14,000 Btu/lb, dry mineral matter-free coal and 2-3% pyritic sulfur, each 100% excess ferric ion consumption results in a 60-100 Btu/lb reduction in the heating value of the coal. In certain coals with relatively high sulfur levels and little or no reaction with the coal matrix, there is an increase in heat content of 1-5% when it is calculated on a dry basis, only because of ash reduction caused by pyrite removal. 

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... 

The results in terms of final sulfur values and pyrite removal are given in Table VIII. Note that pyritic removal computed from either sulfur forms analyses or the diflFerence in total sulfur between processed and untreated coal (Eschka analysis) resulted in essentially identical values of 93-100%. This corresponds to total sulfur removal of 40-70%, depending on the organic sulfur content of the coal. The observation of greater than 100% removal is a result of cumulative error in analysis and the removal of small amounts of sulfate (0.02-0.04%). Presently, these experiments are being duplicated using ferric sulfate, and preliminary analysis indicates the same results. 

The effect of pyrite removal was also clearly demonstrated ( 0). For Burning Star coal (Illinois No. 6), the oil yield for run-of-mine coal (4.8% pyrite) was 47%. Conventional cleaning to 1.5% pyrite resulted in an oil yield of 42%. Deep cleaning to 0.5% pyrite followed by addition of 5.3% pyrite to the clean coal produced an oil yield of 51%, even though the operating pressure (1800 psi) was lower than in the other runs (2250 psi). 

Total 1991 world production of sulfur in all forms was 55.6 x 10 t. The largest proportion of this production (41.7%) was obtained by removal of sulfur compounds from petroleum and natural gas (see Sulfurremoval and recovery). Deep mining of elemental sulfur deposits by the Frasch hot water process accounted for 16.9% of world production mining of elemental deposits by other methods accounted for 5.0%. Sulfur was also produced by roasting iron pyrites (17.6%) and as a by-product of the smelting of nonferrous ores (14.0%). The remaining 4.8% was produced from unspecified sources. 

Fig. 14. Coal flotation flow sheet suggested for increased sulfur removal (29). Pyritic sulfur removal from coal makes it imperative to closely control pulp...

Burning Pyrites. The burning of pyrite is considerably more difficult to control than the burning of sulfur, although many of the difficulties have been overcome ia mechanical pyrite burners. The pyrite is burned on multiple trays which are subject to mechanical raking. The theoretical maximum SO2 content is 16.2 wt %, and levels of 10—14 wt % are generally attained. As much as 13 wt % of the sulfur content of the pyrite can be converted to sulfur trioxide ia these burners. In most appHcations, the separation of dust is necessary when sulfur dioxide is made from pyrite. Several methods can be employed for this, but for many purposes the use of water-spray towers is the most satisfactory. The latter method also removes some of the sulfur... 

Hydrolysis using aqueous alkaH has been found to remove ash material including pyrite. A small pilot plant for studying this process was built at the BatteUe Memorial Institute in Columbus, Ohio (74) and subsequentiy discontinued. Other studies have produced a variety of gases and organic compounds such as phenols, nitrogen bases, Hquid hydrocarbons, and fatty acids totaling as much as 13 wt % of the coal. The products indicate that oxidation and other reactions as weU as hydrolysis take place. 

Biological processes are also being studied to investigate abiHty to remove sulfur species in order to remove potential contributors to acid rain (see Air pollution). These species include benzothiophene-type materials, which are the most difficult to remove chemically, as weU as pyritic material. The pyrite may be treated to enhance the abiHty of flotation processes to separate the mineral from the combustible parts of the coal. Genetic engineering (qv) techniques are being appHed to develop more effective species. 

Nitrogen, unlike pyritic sulfur, is mosdy chemically bound in organic molecules in the coal and therefore not removable by physical cleaning methods. The nitrogen content in most U.S. coals ranges from 0.5—2.0 wt %. 

Conventional coal cleaning processes can remove about 50% of pyritic sulfur and 30% of total sulfur. For northern Appalachian region coals it has been shown that a greater sulfur reduction can be achieved by applying physical coal cleaning to finer size coals (Table 3) (8). 

Oxidative Desulfurization Process. Oxidative desulfurization of finely ground coal, originally developed by The Chemical Constmction Co. (27,28), is achieved by converting the sulfur to a water-soluble form with air oxidation at 150—220°C under 1.5—10.3 MPa (220—1500 psi) pressure. More than 95% of the pyritic sulfur and up to 40% of the organic sulfur can be removed by this process. 

---