Since 1848, Richardson has discovered zinc (Zn) and cadmium (Cd) in coal, except extremely rare polonium (Po), polonium (At), francium (Fr), actinium (Ac) and praseodymium (Pa). Using the existing analytical techniques, 86 elements can be detected from coal and its desorbed samples, and only 88 elements can be counted in the crust (thomas lee, 1992). Scholars at home and abroad classify the elements in coal according to their concentration or content:
-юдович (1978) According to the Clark values of various elements in sedimentary rocks, the elements in coal are divided into ash-making elements (content > 0.5%) and medium elements (content 0.5% ~ 0.01)
—— Cheng Jieke (1986) proposed that the content of elements can be divided into macroelements (1% ~ 100%), trace elements (0.0 1% ~ 1%) and trace elements (0.0.
Generally speaking, the elements in coal can be divided into two categories: macro elements (> 0. 1%) and trace elements (≤0. 1%). The major elements in coal are carbon, hydrogen, oxygen, nitrogen, sulfur, silicon, aluminum, iron, calcium, potassium, sodium and magnesium. And most other elements exist in coal at trace concentrations (Tang et al., 2002a；; Dai Shifeng, 2002).
This book adopts the latter classification. In addition, the elements in coal can be divided into harmful and harmless according to their toxicity.
2. Harmful elements in coal
Harmful elements in coal refer to elements that can be transported to the atmosphere, hydrosphere or PEDOSPHERE in different forms during the processing, utilization, transportation and storage of coal resources, and cause pollution to the environment therein, thus endangering the normal survival and safety of human beings and other organisms.
(1) Types of harmful elements in coal
As we all know, the harmful elements sulfur and nitrogen in coal are very harmful to the environment. However, there is no unified understanding of harmful trace elements in coal, that is, which trace elements have the potential to harm the environment and human health.
—— The National Research Council of the United States divides elements into three categories according to the degree of harm: the first category of pollutants are As, B, Cd, Mo, Hg, Pb and Se; The second kind of pollutants include chromium, copper, fluorine, nickel, vanadium and zinc; These three pollutants are barium, antimony, strontium, sodium, manganese, cobalt, lithium and bromine.
-The American Handbook of Toxic Chemicals lists 29 toxic elements, namely As, B, Ba, Be, Br, Cd, Cl, Co, Cr, Cu, F, Hg, Hf, In, Mn, Mo, Ni, P, Pb, Sb, Se, Sn, Th, Tl and U.
-1990 The Clean Air Supplement Act promulgated by the US Congress lists 1 1 harmful elements, including selenium, barium, cadmium, mercury, arsenic, chromium, lead and silver.
—— Wang Liansheng (1994) ranked the potential toxicity of metal elements, and considered that Class I elements were Hg, Cd, Tl, Pb, Cr, In and Sn, which were highly toxic; The second kind of elements are silver, antimony, zinc, manganese, gold, copper, praseodymium, cerium, cobalt, palladium, nickel, vanadium, osmium, lutetium and platinum, which are moderately toxic.
As far as the types of harmful trace elements in coal are concerned, some organizations and scholars have studied:
-The National Resources Council (NRC) 1980 classifies the elements in coal into six categories according to the degree of harm (Finkelman et al., 1999). Class I is an element worthy of special attention, such as As, B, C, Cd, Hg, Mo, N, Pb, Se and S; Class II is an element worthy of attention, including chromium, copper, fluorine, nickel, antimony, vanadium and zinc. Ⅲ is an element worthy of attention, including Al, Ge and Mn; Class Ⅳ refers to radioactive elements that need attention, such as Po, Ra, Rn, Th, U, etc. ⅴ is an element that needs attention but is rarely enriched in coal and its residues, such as Ag, Be, Sn and TL; Class ⅵ refers to the elements that do not need attention for the time being, that is, elements other than the above five categories (Figure 1- 1).
Figure 1- 1 Potential harmful elements in coal
(According to NRC, 1980)
-Swaine (1995) thinks that there are 24 kinds of trace elements harmful to the ecological environment in coal, namely As, B, Ba, Be, Cd, Cl, Co, Cr, Cu, F, Hg, Mn, Mo, Ni, P, Pb, Sb, Se, Sn, Th and Tl.
-Finchelman (1995) discussed 25 trace elements in coal that are sensitive to the environment, that is, the above 24 elements plus Ag.
-Zhao Fenghua (1997) thinks that there are 19 kinds of elements concerned about environmental protection at present, namely Ag, As, Ba, Be, Cd, Cl, Co, Cr, Cu, F, Hg, Mn, Mo, Ni, Pb, Se and Sb. Among them, thallium, beryllium, cadmium, mercury and lead are toxic elements, while beryllium, cadmium, chromium, nickel, lead and arsenic are carcinogenic elements.
-Pech (1980) classifies the trace elements in coal into six categories according to the degree of harm (Swaine, 1990). Class I elements that need special attention are arsenic, boron, cadmium, mercury, molybdenum, lead and selenium; The second type of elements that need attention are chromium, copper, fluorine, nickel, vanadium and zinc; Class III elements to be noted are barium, bromine, chlorine, cobalt, germanium, lithium, manganese and strontium; Class iv radioactive elements are Po, ra, Rn, Th and u; ⅴ is an element rarely enriched in coal and its residues, such as Ag, Be, Sn and TL; Class VI refers to elements that are basically harmless to the environment, as well as elements other than the above five categories.
-Swaine (2000) thinks that there are 26 trace elements in coal that should cause environmental interest, and divides them into three categories according to their harmfulness, and the degree of harm is reduced from Grade I to Grade III. Class I elements include arsenic, cadmium, chromium, mercury, lead and selenium; The second type of elements are boron, chlorine, fluorine, manganese, molybdenum, nickel, beryllium, copper, phosphorus, thorium, uranium, vanadium and zinc; The third kind of elements include barium, cobalt, iodine, radium, antimony, tin and thallium.
(2) This project mainly studies harmful elements in coal.
The above documents show that the definitions of harmful elements in coal are different, but most of them include As, Be, Cd, Cl, Co, Cr, F, Hg, Mo, Mn, Ni, Pb, Sb, Se, Th and U 16. Although Br does not appear many times in the above literature, it is toxic in itself and has strong corrosiveness to boilers when burning coal.
However, some elements not included in the above literature may also cause harm in the process of coal utilization. For example, the carcinogenesis of rare earth elements is a research topic of great concern, and pulmonary fibrosis caused by long-term inhalation of rare earth dust is called "rare earth pneumoconiosis". Rare earth elements can react with various tissue components in the body through the mouth, respiratory tract and skin. For example, light rare earths can be complexed with amino acids, while heavy rare earths can easily combine with protein. The half-life of inhaled rare earth elements in the body can be as long as one year to more than ten years, and long-term inhalation of rare earth elements is harmful to human body. Workers exposed to rare earth dust particles may often have symptoms such as headache, nausea, cough and allergic fever. One of the most important pathological and biochemical effects caused by rare earths is fatty liver (Chen Qing et al., 1989).
At present, few countries have formulated hygiene standards for rare earth elements. The former Soviet Union put forward that the maximum allowable concentrations of oxide aerosols of various rare earth elements in the air of workplace are Y2 mg/m3, CeO2 5mg/m3, CeO2 6mg/m3 and Yttrium 4mg/m3, and suggested that the maximum allowable concentration of lanthanum in surface water is 0.0 1mg/L/L ... The threshold value of yttrium recommended by the United States in 1960 (TLL Many researchers have concluded that the maximum allowable concentration of rare earth dust is 4 ~ 6 mg/m3, and the allowable intake of rare earth nitrate from food is 12 ~ 120 mg/ (day, person) (Zhao Zhigen et al., 2002a). At present, China is discussing the maximum allowable concentration of rare earth in air, groundwater and food in the workshop of rare earth production and application (Chen Qing et al., 1989). The national standard "Rare Earth Limit in Plant Foods for Human Consumption" (GB13107-91) has been promulgated and implemented.
The United States has studied the rare earth pollution in refineries (Peng An et al., 1995). However, whether the rare earth elements in coal can be included in the category of harmful elements has not been reported in the literature. However, in some coals, especially in coal slime, fly ash and fly ash after burning coal, the abundance of rare earth elements is relatively high, and some even reach or exceed the industrial level (500× 10-6). Peng An et al. (1995) calculated the contribution of different emission sources to atmospheric elements, and found that coal combustion contributed the most to the content of rare earth in urban atmosphere.
In fact, if any element in the periodic table is highly enriched or depleted, it will do harm to the environment and human health (Finkelman et al., 1999), and the elements in coal have their particularity, and their mobility in the utilization process of coal (mainly combustion, washing and leaching) largely determines their harmfulness. Although some elements are toxic in themselves, they are inert in the process of coal utilization and in the process of weathering or rainwater leaching of solid waste after utilization, and basically do not migrate to the external environment, so they are relatively harmless. Some elements, although the coal content is not high, have a large amount of emissions or a small amount of migration in the process of coal utilization, but they can generate more toxic compounds, which are difficult to degrade and accumulate, so they are harmful.
Therefore, the harmfulness or harmlessness of elements in coal is relative. To evaluate its harmfulness, we should not only consider its content level and its own toxicity, but also consider its migration characteristics. At the same time, with the development of science and technology, people's cognitive ability is constantly improving. Elements that are considered harmless at the current cognitive level may be considered harmful in the future. Therefore, it is unscientific to simply limit the types of harmful elements in coal. It is very important to understand their content level, distribution characteristics, occurrence state and migration behavior before, during and after coal utilization. In addition, some elements in coal are interdependent or * * related, so it is one-sided to study some elements alone.
Based on the above considerations, this book not only studies the harmful macroelement S, but also studies the effects of arsenic (as), lead (Pb), mercury (Hg), cadmium (Cd), chromium (Cr), selenium (Se), fluorine (F), chlorine (Cl), nickel (Ni), manganese (Mn) and cobalt in coal. Discuss other harmful or potentially harmful trace elements and elements that are considered harmless at the current level of understanding, that is, under the condition of making full use of existing information, to achieve the goal of not missing or letting go of harmful elements as much as possible.