Mineral matter leaching

For practical applications, the content of inorganic impurities present in activated carbons should be as low as possible. As inert materials they decrease the adsorption capacity of tiie adsorbent. Only for some applications higher ash contents may be beneficial due to the ability of certain ash constituents to chemisorb specific types of adsorbates such as metals, inorganic spiecies and some synthetic organics [337-343]. Furthermore, mineral matter leaching from the carbon can also become a problem of considerable environmental concern. 

More recendy, the molten caustic leaching (MCL) process developed by TRW, Inc. has received attention (28,31,32). This process is illustrated in Eigure 6. A coal is fed to a rotary kiln to convert both the mineral matter and the sulfur into water- or acid-soluble compounds. The coal cake discharged from the kiln is washed first with water and then with dilute sulfuric acid solution countercurrendy. The efduent is treated with lime to precipitate out calcium sulfate, iron hydroxide, and sodium—iron hydroxy sulfate. The MCL process can typically produce ultraclean coal having 0.4 to 0.7% sulfur, 0.1 to 0.65% ash, and 25.5 to 14.8 MJ/kg (6100—3500 kcal/kg) from a high sulfur, ie, 4 wt % sulfur and ca 11 wt % ash, coal. The moisture content of the product coal varies from 10 to 50%. 

The primary solid byproduct of combustion processes is bottom ash, which primarily consists of mineral matter and minor amounts of unreacted carbon. Because the leaching property of the ash, the bottom ash from combustion of most material is considered hazardous. An exception is the bottom ash from combustion of biomass. 

A two-step process for extracting mineral matter and sulfur from coal was demonstrated with three different coals under a variety of treatment conditions. The first step Involves treatment with a hot alkaline solution which extracts part of the sulfur and generally converts much of the mineral matter to an acid-soluble form. The second step Involves leaching with an acid to extract the converted mineral matter. Although H2S0k would likely be used in the second step of a commercial process, HNO, was chosen for the present study in order to shed some light on the disposition of organic sulfur. 

Waugh, A. B. Bowling, K. McG. "Removal of mineral matter from bituninous coals by aqueous chemical leaching," Fuel Processing Techno . 9, 217-233 (1984). 

The pH of the ancient swamp was an important factor controlling mineral matter in coal. A condition of low pH reduces iron to the ferrous state, causes leaching of acid soluble phases, and retards sulfate reducing bacteria, thereby inhibiting fermentation (32). In addition, the pH of the muddy waters that overlie peat deposits may affect the peat in the same manner (40). 

Previous work has shown that ash-forming mineral matter, including iron pyrites, can be removed from coal by leaching the finely divided material with a hot caustic solution under pressure followed by washing with a dilute mineral acid (1-6). Recently, similar results have been achieved in leaching fine-size coal with hot sodium carbonate solutions (J7). In both cases, quartz appears to dissolve in the hot alkaline solution, while clay minerals and iron pyrite are converted into acid-soluble compounds which are removed in the acid washing step. Acid also removes any carbonate minerals which are present in the coal. 

For the leaching experiments, a weighed amount of finely divided coal or mineral matter was mixed with 120 ml. of alkaline solution in a 300-ml. stainless steel (Type 316) autoclave equipped with a turbine agitator. The system was flushed with nitrogen and then heated to the desired leaching temperature. The mixture was stirred continuously while leaching was conducted at constant temperature and pressure for a specified period. After this treatment, the autoclave... 

Unreacted pyrite was not touched by the acids whether hot or cold. Behavior of Mineral Matter During Coal Leaching... 

Most of the mineral matter was removed from a representative coal (Illinois No. 6) by first leaching the finely ground material with a hot alkaline solution and then washing the product with a mineral acid and water. The effectiveness of various alkalis for converting or extracting mineral matter decreased in the following order ... 

Unlike fracturing, this approach should cause only moderate disruption of the coal matrix and might not lead to complete comminution, but this method could be envisioned as a prelude to coal grinding. Removal of material from structural boundaries would yield a more friable, "soft" coal structure. The leaching of fine mineral particles from the coal matrix results in a more porous structure and should also improve the coal s grindability. The prior removal of some mineral matter would also reduce the coal s abrasiveness. 

At the end of the desired contact time, the reactor is removed from the heater, and the hot liquid phase is carefully transferred to a flask. After cooling, the coal is removed from the reactor and washed on a sintered glass funnel with distilled water. Final drying of the coal is carried out in a vacuum oven at 110 C. The cooled liquid phase, which frequently contains a small amount of powdered coal and precipitated mineral matter, is filtered. The water is evaporated to dryness to determine the quantity and nature of minerals leached from the coal. 

Fan, C.-W. Markuszewski, R. Wheelock, T.D. "Behavior of Mineral Matter During Alkaline Leaching of Coal," Am. Chem. Soc. Div. of Fuel Chem. Preprints 1984, 29(4), 319-325. 

The rate at which these elements will leach from the ash sample is dependent on the form in which the element is present and the location of the element within the ash matrix or adsorbed onto the ash particle surface. The ash spheres are chemically stable in the environment and are resistant to weathering due to the aluminosilicate matrix. Any element present in this matrix will not be readily avaiable for leaching. Elements adsorbed onto the surface of the ash spheres will be more readily leached. Uncombusted mineral matter may account for presence of high concentrations of certain elements in the whole ash analyses. However, leachates generated from these ashes may not reflect the high concentrations because the extraneous material associated with the ash is not in a form that is susceptible to leaching within the time span of these experiments. 

Lead enters surface water from atmospheric fallout, run-off, or wastewater. Little lead is transferred from natural minerals or leached from soil. Pb ", the stable ionic species of lead, forms complexes of low solubility with major anions in the natural environment such as the hydroxide, carbonate, sulfide, and sulfate ions, which limit solubility. Organolead complexes are formed with humic materials, which maintain lead in a bound form even at low pH. Lead is effectively removed from the water column to the sediment by adsorption to organic matter and clay minerals, precipitation as insoluble salt (the carbonate, sulfate, or sulfide) and reaction with hydrous iron, aluminum, and manganese oxides. Lead does not appear to bioconcentrate significantly in fish but does in some shellfish such as mussels. When released to the atmosphere, lead will generally occur as particulate matter and will be subject to gravitational settling. Transformation to oxides and carbonates may also occur. 

Heavy rains leach out a portion of the bases in the mull layer and below, most of which are intercepted by the tree roots and returned to the surface again later with the annual leaf fall. Since the leachates are usually nearly neutral in reaction, they extract less mineral matter than where the percolating water is high in organic and nitric acids. This continuous cycling of nutrients accounts for stabilization of the soil with little tendency toward the distinct stratification and impoverishment that is characteristic of true podzols. 

The aqueous caustic recovered from the first coim-tercurrent washing circuit is limed to remove the coal-derived mineral matter, sulfur compounds, and carbonates. The mixture is provided with sufficient residence time to permit precipitation of the impurities before being centrifuged. The purified hquid is preheated and sent to a caustic evaporator where the water is recovered for recycle to the first wash train, while producing anhydrous caustic (as either a molten liquid or as flakes) for reuse in the initial leaching of the coal. 

The inorganic matter within the coal can have a detrimental effect on the process. In addition to poisoning catalysts, there is the potential for leaching harmful elements from residues and ashes after disposal and certain minerals cause abrasive and chemical wear as well as clogging and buildup in the reactors (Harris and Yust, 1977 Russell and Rimmer, 1979). But there is also the suggestion that the mineral matter in coal might have catalytic properties and act as an enhancement to the process (Mukherjee et al., 1972 Bockrath and Schroeder, 1981). 

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