Coal with mineral acid

Humic acids are alkaH-extractable materials and total humic acid content is a term that refers to the humic acid content of coal that has had its carboxylate cations removed with sodium pyrophosphate. Values for some typical AustraHan brown coals range from 24—92% (13). Treatment of lignitic coals with mineral acid to release the alkaH and alkaline cations may dissolve up to 20% of the coal. The naturally moist coals are slightly acidic and have a pH of 3.5—6.5. 

Pyrrole was first isolated in pure form in 1857 from bone oil, although it had been observed in 1834 that coal tar and bone oil contained a substance which imparted a red color to pine splinters moistened with mineral acid. The structural formula was established in 1870. Pyrrole chemistry up to the early 1930s is reviewed in the comprehensive treatise of Fischer and Orth (B-34MI30400) and two modern (excellent) books by Gossauer, and Jones and Bean are available (B-74MI30400, B-77MI30400). 

Cadmium is found naturally deep in the subsurface in zinc, lead, and copper ores, in coal, shales, and other fossil fuels it also is released during volcanic activity. These deposits can serve as sources to ground and surface waters, especially when in contact with soft, acidic waters. Chloride, nitrate, and sulfate salts of cadmium are soluble, and sorption to soils is pH-dependent (increasing with alkalinity). Cadmium found in association with carbonate minerals, precipitated as stable solid compounds, or coprecipitated with hydrous iron oxides is less likely to be mobilized by resuspension of sediments or biological activity. Cadmium absorbed to mineral surfaces (e.g., clay) or organic materials is more easily bioaccumulated or released in a dissolved state when sediments are disturbed, such as during flooding. 

In order to ensure a consistent supply of coal for steam generation, plants typically maintain an outdoor 90-day reserve supply. The pUes are usually not enclosed, so the coal comes in contact with moisture and air, which can oxidize metal sulfides to sulfuric acid. Precipitation then results in coal pile runoff with minerals, metals, and low pH (occasionally) in the stream. 

Hatchett treated various kinds of wood, coal, and coke with nitric acid and found that a substance very analogous to tannin. . . may at any time be produced by exposing carbonaceous substances, whether vegetable, animal, or mineral, to the action of nitric acid. He also converted skin into leather by means of materials which, to professional men, must appear extraordinary, such as deal sawdust, asphaltum, common turpentine, pit coal, wax candle, and a piece of the same sort of skin.. . . ... 

At NBS, the neutron activation with combustion separation method used for determining mercury in coal (12) was further investigated for determining selenium by Rook (13). The same procedure is used except that the sample is heated finally to 1000°C. Mercuric oxide is also used as carrier for the selenium because selenium oxides are difficult to dissolve in mineral acids. The mercuric selenide formed carries the selenium effectively, and, as it is soluble in nitric acid, the dissolution procedure developed for the mercury separation can be used so that mercury and selenium can be determined in the same sample. 

One particular test method (ASTM D-1756) covers the determination of carbon dioxide in coal in any form, such as mineral carbonate, from which carbon dioxide is released by action of mineral acids (e.g., hydrochloric acid). The method can be applied to high- and low-carbonate coals. The determination of carbon dioxide is made by decomposing with acid a weighed quantity of the sample in a closed system and absorbing the carbon dioxide in an absorbent (e.g., such as sodium... 

A reliable method of measuring the mineral matter content of a coal is an acid demineralization procedure. The method depends on the loss of weight of a sample when treated with 40% hydrofluoric acid at 50 to 60°C (122 to 140°F). Treatment of the sample with hydrochloric acid before and after treatment with hydrofluoric acid helps prevent the retention of insoluble calcium fluoride (CaF2) in the coal. Pyrite is not dissolved in the treatment, consequently, pyrite and a small amount of retained chloride must be determined separately. Since two-thirds of the mass of the pyrite (FeS2) is accounted for by the presence of ferric oxide (Fe203) in the residual ash, the mineral matter content is then given by the formula... 

Three methods for determining mineral carbon dioxide in coal were investigated using bituminous coal. The titrimetric method is claimed to be superior to either of the then-used British standard gravimetric or manometric methods (BS 1016). The procedure involves the decomposition of carbonate minerals with hydrochloric acid and absorption of the evolved carbon dioxide in a mixture of benzylamine, ethanol, and dioxan. This mixture forms a stable salt of benzylcar-bamic acid, which is then titrated with sodium methoxide. The method was said to be suitable for all concentrations of carbon dioxide. It is especially accurate for low concentrations, and it is much more rapid than other methods tested. 

Analytical values for the eight coals after treatment with 2 M HN03 are given in Table III. The reported values are the averages of four determinations (duplicate determinations on different days). A comparison of the dry ash values for the HNOj-extracted residues described in Table III to the dry ash values for the raw coals described in Table I reflects the reduction in mineral matter caused by extraction of the raw coals with 2 M HN03. Carbonates, sulfates, and other minerals dissolve in the acid solution used to extract pyrite. 

Various bituminous coals were demineralized by an experimental two-step leaching process in which the ball-milled coals were first treated with a hot alkaline solution and then with a dilute mineral acid. Different alkalis and acids were studied to determine their relative effectiveness. In addition, the effects of alkali concentration, treatment temperature, and treatment time were evaluated. Under the best conditions, the process reduced the ash content of the coals by 85-90% and the total sulfur content by 70-90%. As the temperature of the alkaline treatment was raised from 150 to 345 C, the removal of sulfur increased greatly whereas the recovery of organic matter declined. When a 1 M sodium carbonate solution was employed for the treatment, the recovery of organic matter was 91-97% for various coals treated at 250 C and 79-89% for the same coals treated at 300 C. 

The results of leaching ground raw coals with HN03 alone are indicated in Table I. The sulfur content of the acid-leached coal is indicative of the organically bound sulfur, while the ash content reflects the removal of iron pyrite and other minerals such as carbonates which are soluble in nitric acid. It can be seen that acid leaching alone reduced the sulfur content of Cherokee coal by 57%,... 

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. 

Thirty-four minor and trace elements are of potential environmental concern (n ). Sulfur is the element of major concern due to its abundance in flue gases from some coal-burning plants and its subsequent contribution to "acid rain." Sulfur as acidic ions of sulfate can also contribute to pollution of surface water and groundwater. Other elements of greatest concern are As, B, Cd, Pb, Hg, Mo, and Se. With the exception of B and Se, these elements are strongly associated with mineral matter in the coal and are concentrated in waste piles from coal preparation plants. If the waste disposal site is not constructed as a closed system, pollution of nearby groundwater is possible. Boron and Se may contribute to the pollution risk as they are associated with both mineral and organic components. On the other hand, certain coal-mine wastes have potential for recovery of valuable metals such as zinc and cadmium (18). 

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. 

---