Coal is mainly a solid combustible mineral formed by plant residues, both organic and inorganic, and almost contains various elements of common minerals in the earth's crust. At present, more than 60 elements related to coal have been found. Although the content of these associated elements is not high, the average content of most of them exceeds the average content (Clark value) of this element in the crust. The term trace elements or trace elements in coal originated in the middle of19th century. Because the existence of trace or trace chemical elements can only be qualitatively determined when the elemental composition in coal is determined by spectral analysis, it is called trace or trace elements.
The accumulation of trace elements in coal depends on the elemental composition of original coal-forming substances, the characteristics of coal-forming environment and various physical, chemical and geochemical actions experienced during and after coal-forming period. According to the relationship between the associated trace elements in coal and the coal-forming process, the sources of trace elements are various. Some trace elements exist in the living state of coal-forming plants and then are brought into coal. Some elements accumulate in peat bogs after the death of coal-forming plants and are brought into mineral impurities by external forces (such as wind, water, atmospheric precipitation, etc.). ). After the coal-forming material forms peat and is buried, some trace elements are leached by water and infiltrated into coal from overlying strata along pores and structural fractures during coalification. After coal formation, some trace elements were brought into the coal due to the intrusive contact of igneous rocks, volatile gas and hydrothermal activity in the later period.
Many research results show that organic matter has always played an important role in the enrichment of associated trace elements in coal. Among them, the decomposition process of plant residues in swamp plays a more prominent role in the migration and enrichment of elements. The enrichment of trace elements in coal is basically chemical and physical adsorption, that is, humic acid and humus formed by the decomposition of coal-forming substances have high adsorption capacity, which is beneficial to the enrichment of trace elements in the early stage of coal formation. In addition, coordination can change the migration and enrichment ability of some trace elements, that is, they can all be used as ligands to coordinate with metal ions because they contain functional groups such as amino, hydroxyl and carboxyl, and organic substances such as humus in peat bogs. Metal-organic complexes formed by elements, some insoluble in water, enriched due to reduced mobility, and some soluble in water, greatly increase the mobility of elements. For most trace elements, their concentrations in coal-forming swamps and other water bodies are generally very small, and it is almost impossible to precipitate out of the solution because they exceed their solubility products. Therefore, the adsorption of humus as adsorbent has become the basic way of element migration and enrichment.
The distribution of trace elements in coal seams and between coal seams is determined by many geological and geochemical factors.
The content of trace elements in coal obviously depends on the coal-forming environment, especially the change of pH value and Eh value of the environment, which not only affects the difference between coal and rock components and swamp environment, but also affects the aggregation and dispersion of trace elements.
Trace elements and organic matter have different affinities. Some metal elements with high ionic potential, such as beryllium, germanium, uranium, zirconium, etc., can almost completely combine with organic matter in coal, have strong organic affinity, and are mainly enriched in the cementitious components of coal. According to the data of Mincev (1972) about the composition of lignite and the contents of some trace elements, compared with raw coal, wood coal is rich in vanadium, manganese, strontium and barium, vitrinite is rich in lead and nickel, gel is rich in cobalt, arsenic, silver, molybdenum, germanium and tin, and silk is rich in yttrium, beryllium, ytterbium and tin. The enrichment of elements by sericite is mainly mineralization, but the enrichment of yttrium and ytterbium may be related to organic matter.
The enrichment of trace elements in coal is also affected by contact metamorphism and regional metamorphism. Contact metamorphism is often caused by the intrusion of younger igneous rocks, and most of them are accompanied by mineralization. Due to the role of volatile gas and hydrothermal solution, some trace elements are enriched. Regional metamorphism obviously affects the enrichment of boron, germanium and other elements, and the content of trace elements decreases with the increase of metamorphic degree. This is because mineral components (carbonate, oxide and other salts) can be separated from organic compounds such as humic acid in the process of coalification, while soluble components (such as GeO2 or Na2GeO3) are carried out of the coal seam by circulating water.
The enrichment of many trace elements is often related to the strata of coal seam, which is often concentrated in the strata near the roof, floor and dirt band of coal seam. Among them, the influence of geological age (horizon) of coal-bearing strata on trace elements is mainly caused by the irreversible changes of various geological processes, the evolution of ancient plants, the characteristics of paleoclimate and paleoenvironment with the evolution of geological history, but the specific influence mechanism, the scope and degree of influencing trace elements are still to be studied. Some people think that the enrichment mechanism of trace elements in coal seam near the roof and floor and gangue is formed by more minerals and mineral-rich solutions entering the swamp before and after peat swamp accumulation than in other periods.
Many researchers believe that the enrichment of trace elements near the roof, floor and gangue of coal seam is due to the enrichment of trace elements from the surrounding rock or peat layer of coal seam through diffusion and infiltration during or after diagenesis, forming an enrichment zone near the surrounding rock coal seam, so it is called contact zone enrichment, and the diffusion in coal seam is mainly carried out in gel-like substances. This understanding further explains why trace elements are easy to be enriched in thin coal seams and lenticular coal seams.
The enrichment of associated trace elements in coal is closely related not only to organic matter, but also to inorganic ash. Some trace elements combine with minerals with certain genetic types (that is, trace element carriers) after entering coal seams or peat beds. The more this mineral, the more trace elements are accumulated in coal.
In addition to the above factors, the paleogeographic conditions of coal seam formation also affect the enrichment of trace elements. For example, trace elements tend to be enriched in coal near the edge of coal-bearing basin or provenance; Some trace elements are high in continental coal basins and low in offshore coal basins, such as germanium in coal. In addition, the enrichment of trace elements in some coals is related to contemporaneous or quasi-contemporaneous magmatic and volcanic activities.
Trace elements in coal can be used as useful mineral resources if they are enriched to the content suitable for development and utilization. With the continuous expansion of the application scope of coal, more and more problems are involved in the study of trace elements in coal. For example, coal quality evaluation, coal washing and processing, coal gangue treatment and cinder harmless treatment, comprehensive utilization of coal, environmental protection, and research on the role of trace elements in coal gasification and liquefaction. In the study of coalfield geology, the study of trace elements in coal provides methods and basis for coal seam correlation, coalification research and coal-forming environment analysis.
When coal is processed and utilized, trace elements in coal will be converted into liquid hydrocarbon products, coke and other products. In the complex transformation process of coal, sometimes trace elements can act as catalysts or inhibitors, but some elements will be released into the surrounding environment due to coal combustion or wind oxidation of coal and cinder, and some elements will become toxic substances to animals, plants and human beings through certain forms. According to American data, some radionuclides formed by coal combustion, such as uranium, thorium, tritium, argon, inert gas, iodine, radon, polonium and other isotopes, can easily become human carcinogens.
With the development of nuclear industry and electronic industry, the demand for rare metals has increased rapidly, and many countries have carried out research and utilization of trace elements in coal. Recently, toxic, radioactive and corrosive trace elements in coal have been widely studied. Germanium, uranium and vanadium are the most abundant trace elements associated with coal in China, which are listed as follows.
Two. uranium
Uranium is the main raw material of modern atomic energy industry, and uranium associated with coal is one of the important types of this deposit. The industrial grade requirement of associated uranium in coal is generally 0.02%.
At present, most of the known uranium-rich coal seams with industrial value are formed in continental sedimentary environment, especially lignite beds. Most of these uranium-rich coal seams are located on the basement crystalline rocks in the coal basin, and some of them are interbedded with acidic extrusive rocks. In addition, the bituminous shale in shallow sea or the stone coal in China also generally contains uranium, which often coexists with phosphorus, vanadium and other elements. This kind of uranium deposit has large reserves, but low grade.
Uranium mainly exists in coal in the form of organic compounds of uranium. During the peat accumulation period, most of them migrate in the form of soluble uranium organic complexes and in different humate complexes. When humic acid is oxidized, the complex is destroyed, or uranium organic complex reacts with some salts, or precipitates due to adsorption; Uranium can also migrate in the form of colloidal uranium, and can also be precipitated by the reduction of organic matter.
In the stage of peat accumulation and coal formation, organic matter has obvious enrichment effect on uranium. The humic acid solution formed by decomposition of plant residues can decompose the complex of uranium entering swamp water to form uranyl ions, and form uranyl humate through adsorption, ion exchange or coordination chelation. In the coal-forming stage, due to the decrease of Eh value, uranyl ions adsorbed, complexed or exchanged with humic acid ions are desorbed and reduced and precipitated into uranium-rich bodies.
Denson( 1959) put forward three hypotheses about the accumulation and formation of uranium in coal, namely: primary uranium refers to the accumulation of plants living in swamp water or dead organic matter from surface water before coalification; Diagenetic uranium refers to uranium or uranium-bearing deposits at the edge of coal basin brought into coal by water during coalification; Supergene uranium refers to groundwater formed by hydrothermal or unconformity overlying volcanic rocks after coalification and consolidation of surrounding rocks.
Uranium is mostly concentrated near the roof and floor of coal seam, and its content gradually decreases towards the center of coal seam. The uranium content mostly decreases with the increase of coal ash. Breg and Sopf (1955) studied the bituminous coal seams and lenses of Upper Devonian in Tennessee and Ohio, USA. The contents of coal ash and uranium are shown in table 1 1-2.
Table 1 1-2 Relationship between coal ash and uranium content
(According to Breg et al., 1955)
This phenomenon of low ash content and high uranium content is mainly due to the adsorption of uranium carried by water by humic acids during deposition and coalification. Breger et al. (1955) think that uranyl carbonate complex of alkali metal or alkaline earth metal is unstable in water, and forms uranyl ion UO2+ in acidic environment, and forms uranyl organic complex with organic components in coal.
The relationship between uranium content and coal composition often indicates that there are many cementitious components with high uranium content. In each coal seam of coal-bearing measures, uranium is mostly concentrated in the bottom coal seam of coal measures. In many lignite basins in Yunnan Province, China, uranium is relatively enriched in the coal seam at the bottom of the basin. For example, in five layers of coal in Tian Yang lignite basin, the average uranium content of coal samples from bottom to top is 2 1.7× 10-6, 17.6× 10-6,165,438+0.9 respectively.
Three. germanium (Ge)
Germanium is a rare dispersed element, which mainly occurs in coal seams as an associated element. Generally, the content is not high, and it can be processed and utilized when it reaches 20g per ton of coal. The process of extracting germanium from coal ash, coal ash and other coal processing products is relatively simple and has become one of the important sources of germanium.
The distribution of germanium in coal seam is often concentrated near the roof and floor; In addition, the thin coal seam and lens are rich in germanium.
The main occurrence States of germanium in coal are humate, adsorbed or other germanium organometallic compounds, silicates or sulfides and germanium-containing oxides.
The enrichment of germanium in coal depends on the supply of germanium and sufficient humic acid in the process of coal formation. Humic acid has a large number of active functional groups, large surface adsorption capacity and strong ion exchange ability. Germanium is an extremely active element in supergene geochemical activities, and it is most easily captured by humic acid in coal to form humic acid complexes. There is excess humic acid in peat swamp, so the enrichment of germanium in coal mainly depends on whether there are rich germanium ions or their compounds in the medium of coal basin. Usually, various granites, granite gneiss, basic-acid igneous rocks and mixed metamorphic rocks have high germanium content, forming germanium-rich parent rocks; Secondly, the migration of germanium from the parent rock lattice is determined by the weathering difficulty of the rock itself and the weathering conditions such as structure and hydrology. In the structurally stable area, the rocks are mainly chemically weathered, which is beneficial to the migration of germanium. It can be seen that the relatively stable and slow subsidence environment is beneficial to the enrichment of germanium in coal. In the process of coal formation, favorable climate and rainfall, obvious temperature difference change and strong solubility of surface water in the source area are favorable hydrogeochemical conditions for germanium enrichment in coal.
Germanium is one of the elements with strong organic affinity. Generally, the higher the vitrinite content in coal, the higher the germanium content. The silk component has poor adsorption of germanium, which is due to the lack of humic substances with high adsorption capacity in the silk component during coal formation. Because germanium and organic matter coexist, the distribution of germanium in coal also shows the phenomenon of low ash and high germanium content.
The distribution of germanium in coal seam tends to increase from the middle of coal seam to the outer edge, and it is often concentrated near the top and bottom of coal seam, as well as in thin coal seam and coal lens in sandy and clayey rocks. The enrichment of germanium is also related to the lithology of the roof and floor of coal seam. The germanium content of coal seam near sandstone is higher than that of coal seam near clay rock.
The content of germanium is often related to the stratigraphic age of coal seam. The coal seam is young with high germanium content. This is not only because the enhancement of coalification affects the enrichment of germanium in coal seam, but also because the surrounding rock has undergone corresponding diagenetic metamorphism, which reduces the porosity and affects the seepage of solution.
Four. vanadium
The distribution of vanadium is quite dispersed, and most of it is associated with other elements to form vanadium-bearing deposits. According to the regulations of industrial departments, the content of V2O5 is 0.5% at the cut-off level and 0.7% at the industrial level. Therefore, rocks with V2O5 content of 0.5% ~ 1% are called vanadium ores, and rocks with V2O5 content greater than 1% are called vanadium-rich ores. The Early Paleozoic "Stone Coal" in China is a sapropelic anthracite with siliceous mud, in which the vanadium content is very high, some as high as 1. 18%.
The enrichment of vanadium in sedimentary rocks is closely related to organic matter. The research by Zhang Aiyun et al. (1987) shows that the V2O5 content increases with the increase of plankton skin content, showing a good positive correlation. In some tunicates, the shell of plankton is the outer wall of organic matter. This zooplankton can gradually accumulate only one millionth of vanadium in seawater in the body and bury it in sediments after death, so vanadium is enriched in seabed sediments.
The occurrence of vanadium is mainly enriched in organic matter, clay minerals and independent vanadium minerals.
Vanadium in China stone coal is mostly concentrated in the stagnant water reduction environment, mostly located in the limited basin of continental shelf sea, marginal sea slope and marginal sea basin.
Among the stone coal in China, the horizon, thickness and grade of vanadium deposits are symmetrically distributed, and most of them are concentrated in the ore-rich horizon. The ore-rich horizon is formed in the transition stage from regressive mini-cycle to regressive mini-cycle, which is related to calcium content, coarse and fine clastic content and phosphorite content.
Five, other trace elements in coal
Besides the rare elements mentioned above, coal is sometimes rich in trace elements, such as beryllium, lithium, rubidium, rhenium, indium, thallium, thorium, titanium, niobium, tantalum, zirconium, strontium, tungsten, silver, gold and platinum. Because of their wide application, people pay more and more attention to them. For example, beryllium plays an important role in atomic energy, rockets, missiles, aerospace and electronics industries. Some people call it space metal.
Beryllium is mainly organically bound in coal and closely related to vitrinite. Beryllium content in coal is not high, reaching (10 ~ 20) × 10-6, and some are as high as 40 × 10-6. Beryllium-bearing coal seams are generally distributed at the edge of coal-accumulating basins. In the process of coal utilization, beryllium in coal has a great influence on the surrounding environment. It is a highly toxic element, which can cause cancer, so countries all over the world attach importance to the recovery and comprehensive utilization of beryllium in coal.
Lithium can also be enriched in coal seams. Lithium oxide LiO2 _ 2 is not volatile, so lithium in coal ash is more abundant. Because lithium is mainly used in aerospace, atomic energy, military and chemical fields, the research on extracting lithium from coal has attracted much attention.
Elements such as strontium and rubidium are also found in coal, but the rubidium content is very small, and the strontium content can reach (20 ~ 50) × 10-6.
Rhenium can be used as the material of high temperature resistant components of spacecraft, and alloys of rhenium, tungsten and thorium can be used as electron tube components. When the enriched grade of rhenium in coal reaches above 2× 10-6, it has industrial extraction value. The grade of rhenium in coal in China is low, mostly below 1× 10-6.