Comprehensive utilization of coal gangue is to eradicate gangue
1. Project background and significance
1.1 Project background and significance
Coal gangue is a solid waste discharged during coal mining and coal washing. It is a black-gray rock with low carbon content and harder than coal that accompanies coal seams in the process of coal formation. The coal gangue accumulated in our country's coal system over the years has reached more than 5 billion tons, and the annual emissions are equivalent to about 10% of the coal production in that year. At present, there are more than 1,500 coal gangue mountains in our country, covering an area of more than 200,000 mu. The pyrite contained in coal gangue is easily oxidized by air, and the released heat can promote the spontaneous combustion of coal gangue. When the gangue burns, it emits an unpleasant smell and harmful smoke, which increases the number of patients with chronic bronchitis and asthma in nearby residents. The surrounding trees lose leaves and crops reduce production. The coal gangue mountain is initialized by rain, which often makes the riverbed of the nearby river silt and the river water is polluted.
Coal gangue is also a resource that can be used. Its main components are Al2O3 and SiO2, and it also contains different amounts of Fe2O3, CaO, MgO, Na2O, K2O, P2O5, S and trace amounts of scattered elements (such as gallium, etc.). Common mineral components are: clay minerals, carbonate minerals, bauxite, pyrite, quartz, mica, feldspar, carbonaceous and plant fossils. When a certain element or several elements of gangue are enriched to have industrial utilization value, it can be extracted and utilized. Some coal gangue can be used to burn lime instead of fuel; it can produce ordinary Portland cement, special cement and clinker-free cement; it can be used to produce coal gangue sintered bricks and coal gangue light aggregates (light aggregates are a type of porous aggregate selected to reduce the relative density of concrete); it can produce coal gangue cotton; some can be used to produce chemical products such as aluminum salts (such as polymeric aluminum sulfate and other water purifiers); some can be used to make water glass, silica, alumina, gallium chloride, etc. The use of water glass (sodium silicate, sparkine) is almost all over the national economy. In the chemical system is used to make silica gel, white carbon black, zeolite molecular sieve, sodium metasilicate, silica sol and instant powder cavorine, potassium sodium silicate and other silicate products, is the basic raw material of silicon compounds. Precipitation silica is a general term for white powdery amorphous silicic acid and silicate products, and its composition can be expressed by SiO2 · nH2O. Precipitation silica is high temperature resistant, non-flammable, odorless, odorless, has good electrical insulation, mainly used as a reinforcing agent for natural rubber and synthetic rubber, etc. Polymer aluminum sulfate is a new type of high-efficiency inorganic polymer coagulant, with less impurities, high molecular weight, good coagulation effect characteristics, can be used to purify domestic water, factories, mines, oil fields and other wastewater treatment. In the oil refining industry, it is used for oil-water separation, and the effect is very good. Gallium accounts for about 1.5/100,000 in the earth's crust, and the amount is not small, but it does not form a concentrated individual metal ore, and extraction is very difficult. Even the richest black stone contains only 0.1-0.8% gallium. Gallium is enriched in coal gangue, such as the coal gangue in Yunhuqiao mining area in Xiangtan, which contains gallium up to 24.1g/t. Gallium chloride is mainly used as a catalyst for chlorination reaction in organic synthesis; gallium-rich solution electrolytic deposition or chemical replacement can produce metal gallium. Gallium is a good component of fusible alloys. Pure gallium and its fusible alloys can be used as heat exchange media in nuclear reactors, as high temperature thermometers or fire protection signal devices, etc. Using gallium-aluminum alloy instead of mercury to make medical ultraviolet radiation lamps can improve medical results. The production of semiconductor materials is the main use of gallium.
The comprehensive utilization of coal gangue is supported by national policies. State Economic and Trade Commission, Ministry of Science and Technology "Technical Policy Points for Comprehensive Utilization of Coal Gangue" National Economic and Trade Resources [1999] No. 1005 Notice, comprehensive utilization of coal gangue is a long-term technical policy; recycling beneficial components and producing chemical products is one of the policy points. Therefore, it is of great significance to carry out comprehensive utilization of coal gangue in depth, turn waste into treasure, and produce high value-added chemical products.
1.2 Domestic technology status quo and development trend
1. Domestic research status
Comprehensive utilization of coal gangue is the most thorough way to eradicate disaster accidents in coal gangue mountains. In recent years, our country's coal mines, especially state-owned key coal mines, have achieved certain achievements and experience in comprehensive utilization of coal gangue.
① Utilizing coal gangue for power generation: our country has successfully developed a circulating fluidized bed boiler with coal gangue and coal slime mixed combustion, which solves the technical problems of mixed combustion of different low calorific value fuels. At present, the annual power consumption of gangue in gangue power plants across the country accounts for about 60% of the comprehensive utilization of gangue.
② Using coal gangue to make bricks: At present, the technology of coal gangue brick making machine, furnace design and production equipment in our country are basically close to the level of developed countries. For example, Shandong Xinwen Mining Group and other units have built more than 20 coal gangue brick production lines, and the capacity of each production line is more than 60 million pieces/year.
③ The use of coal gangue to fill subsidence areas, road construction, soil and water conservation, crop planting and other technologies have also made great breakthroughs. For example, Shandong Xinwen Mining Group has used a total of more than 2 million tons of coal gangue to control sand pits and some mine mining subsidence areas on both sides of the Chaiwen River flowing through the mining area, and reclaimed more than 500 acres of development land.
④ Some kaolinite with high content and purity can be used as active fillers for plastics, rubber, paints, coatings, and other polymer materials and composites through calcination and surface modification processes.
The chemical composition of coal gangue and clay is similar, and it can replace clay to provide siliceous and aluminum components as raw materials and fuels for the production of Portland cement and special cementing materials.
(6) Coal gangue is rich in silicon and aluminum, as well as some iron and a small amount of titanium. Through different processes and appropriate ingredients, a series of Al-Si-Fe, Al-Si-Ti alloys can be produced. Alloys of silicon-aluminum-iron alloys are the main deoxidizers in steelmaking. Alloys produced with coal gangue also contain gallium, vanadium, rare earth and other elements, which can significantly improve the casting, welding and extrusion rolling properties of the alloys.
Overview of methods for extracting chemical products from coal gangue
(1) Extraction of alumina from coal gangue
Alumina is the basic raw material for the production of electrolytic aluminum, and it is also an important chemical product. At present, bauxite is mainly used as raw material for the production of alumina at home and abroad. However, with the increasing demand for non-ferrous metals at home and abroad and the exhaustion of bauxite resources, using coal gangue as a backup resource for extracting alumina not only provides an effective way for the sustainable development of the alumina industry, but also improves the comprehensive utilization value of coal gangue. The more commonly used methods for extracting alumina from coal gangue are: acid dissolution method and alkali method. The alkali method is divided into soda lime sintering method and limestone sintering method. Lv Shuzhen and others use limestone sintering method to treat coal gangue containing Al2O325.30% and SiO245.12%, and the leaching rate of aluminum components can reach more than 80%. Liu Xiaobo et al. treated coal gangue containing Al2O330.22% and SiO251.7%, using coal gangue-limestone-soda ash sintering method, the extraction rate of alumina was as high as 85.2%. Zhao Yin et al. treated coal gangue by acid leaching method, crushed the coal gangue into a granularity of -80 mesh, and then activated sintering at 700 ° C for 1.5h. The sintering material was leached with hydrochloric acid, and the leaching rate of alumina could reach more than 85%. Meng Xianmin et al. treated coal gangue by acid leaching first and then alkali decomposition, and amorphous aluminum hydroxide was obtained, and alumina was obtained by high temperature calcination. Alkali method is mostly used in industrial production of alumina, and the acid method is expensive and requires corrosion-resistant equipment, and the economic benefits are not obvious.
(2) Extraction of silica from coal gangue
Silica is an amorphous silica with a white and transparent appearance, light weight, and fluffy powder like fluff. It is an important fine chemical product and is widely used in various fields such as pharmaceuticals, rubber, plastics, and daily chemical products. Liu Xinmei et al. prepared silica from coal-measure kaolin by acid washing, roasting activation, and two-stage acid dissolution. The specific surface area of the obtained silica is 529.6 m2/g, and the mass content of silica is 96.2%. Chen Weixiong used coal gangue as raw material to prepare silica, mixed the crushed coal gangue with soda ash, melted at high temperature, leached in water, filtered, washed, and concentrated to obtain sodium silicate, and then mixed the sodium silicate into a water glass solution, stirred with sulfuric acid, and then filtered, washed, dried, and sorted to obtain silica.
(3) Extraction of polyaluminium ferric (PAFC) from coal gangue
High-sulfur coal gangue is prone to spontaneous combustion, and the coal gangue formed after spontaneous combustion is called spontaneous combustion coal gangue. Extracting polyaluminium iron from spontaneous combustion coal gangue opens up a new way for the comprehensive utilization of spontaneous combustion coal gangue. Guo Zhongquan and others used hydrochloric acid leaching to extract polyaluminium iron from spontaneous combustion coal gangue. The product Al2O3 content is 33%, Fe2O3 content is 16%, the basicity is 70%, and the product extraction rate is 19.85%.
(4) Synthesis of 4A molecular sieve
Molecular sieve is a kind of artificial synthetic zeolite, which belongs to the category of aqueous shelf-like aluminosilicate. The traditional method of molecular sieve production and synthesis using chemical raw materials. Due to the high production cost, it hinders the expansion of the application range of molecular sieves. The synthesis of molecular sieves by alkali treatment with mineral raw materials such as coal gangue (mainly kaolinite) has strong competitiveness due to its rich, cheap raw materials, simple process and low cost. Liu Dameng and others synthesized 4A molecular sieves by using coal gangue containing SiO253.06% and Al2O3 28.66%. Gu Bingwei also prepared 4A molecular sieves by calcining coal gangue with kaolin content above 90% under the action of strong alkali.
(5) Extraction of gallium from coal gangue
Gallium is a dilute element and is mainly used in the semiconductor industry. For coal gangue with high gallium content, especially when the gallium grade reaches 60g/t, its comprehensive utilization should be centered on the recovery of gallium, while taking into account other useful components of the coal gangue (mainly aluminum and silicon). Liu Guangyi et al. mainly use two processes to extract gallium from gallium-rich coal gangue: one is high-temperature calcination leaching, and the other is low-temperature acidic leaching. High-temperature calcination, the leaching rate of gallium and aluminum can reach more than 85%. Low-temperature acidic leaching, the leaching rate of gallium is less than 75%. Gallium can be further extracted from the above acid leaching solution by solvent extraction method, and after multi-stage continuous countercurrent extraction, gallium can be enriched more At last, gallium can be prepared from stripping solution by electrolytic deposition or chemical replacement.
2. Current status of foreign research
Countries around the world are paying more and more attention to the treatment and utilization of coal gangue, and have formulated some protective policies. The US government has encouraged the development of coal gangue power generation and land reclamation through a number of policies, and strictly implemented the Open-pit Mining Control and Restoration Law (promulgated in 1970), requiring that mining rights must be accompanied by corresponding reclamation tasks, and coal companies must pay environmental protection reclamation deposits. After mining, the experience will be observed for 5-10 years, and then returned after confirmation.
The main measures for the utilization of coal gangue in Germany are: one part is used to fill the underground goaf by wind, and the other part is processed and screened as a building material.
In Russia, in addition to being used as filling materials for underground goaf and for road engineering and construction materials, organic mineral fertilizers are also produced from coal gangue with more than 20% organic matter, which can stabilize crop production by 15% to 40%.
The Polish cement industry uses coal gangue from Heldex Company as cement raw materials. There are many advantages to using coal gangue as cement raw materials: gangue contains combustible substances with a calorific value of about l000 × 4.19 - 1500 × 4.19 KJ/kg, which can reduce fuel consumption by about 10%; gangue contains iron oxide flux, which can reduce the firing temperature of clinker during calcination and form a glass layer on the kiln lining, which plays a protective role, prolongs the life of the kiln lining, reduces the consumption of refractory materials by 10% to 20%, and increases the operating time of the kiln. In conclusion, the degree of coal gangue utilization varies from country to country depending on the composition, environmental requirements, and policies of various countries. At the same time, some protective policies have also been formulated. Typical is the government's requirement for clean energy and legislation.
3. Development trends
Since the 1960s, the comprehensive utilization of coal gangue has attracted great attention from various countries. Western countries such as the United States and the United Kingdom have already developed corresponding countermeasures. The utilization rate of coal gangue in these countries has reached more than 90%. In our country, the comprehensive utilization rate of coal gangue was less than 43% in 2000. By the end of 2007, the comprehensive utilization of coal gangue reached 140 million tons, and the utilization rate only reached 50%. Most of them are used in low value-added fields such as power generation, clay bricks and ground fillers. At present, bauxite resources are relatively abundant, and the economic benefits of extracting alumina or other aluminum-containing compounds from coal gangue alone cannot be compared with those extracted from bauxite. However, with the increasing demand for non-ferrous metals in the international market and the continuous depletion of bauxite resources, it is of great significance to utilize coal gangue as a reserve aluminum resource. The process of extracting a single chemical product from coal gangue is economically unadvantageous, and the amount of waste residue is large and the environmental cost is high.
Coal gangue contains dozens of elements, and the research on its comprehensive utilization and turning waste into treasure has attracted extensive attention. The main chemical elements of coal gangue are silicon, oxygen, aluminum, etc. The separation and extraction of silicon and aluminum products from coal gangue is the focus of comprehensive utilization of coal gangue. Recycling chemical products and extracting loose elements from coal gangue has high ecological and economic benefits. The development of deep processing products of coal gangue, especially the comprehensive development of high value-added chemical products (including the extraction of loose elements) is the development trend of comprehensive utilization of coal gangue.
1.3 The existing research foundation and conditions for the realization of this project
The Changsha Mineral Resources Supervision and Testing Center of the Ministry of Land and Resources has raised 100,000 yuan of its own funds and has carried out the preliminary development of the project, such as sampling and analysis of coal gangue in some mining areas, and the preparation of water purification agents from coal gangue. The test products have been tested, and the water purification effect is comparable to that of the water purification agents used in the current water works. The quality of water glass and silica meets the industry first-class standards. Through the implementation of this project and the foundation of several years of dedicated research, the project application team strives to achieve environmentally friendly and efficient comprehensive utilization of coal gangue, and achieve short production cycle, less pollution of "three wastes" and zero environmental pollution.
2. Overall objective
In order to comprehensively utilize coal gangue resources, extract chemical products from coal gangue, and maximize economic and environmental benefits, it is necessary to comprehensively utilize aluminum, silicon, carbon and gallium in coal gangue.
The goal of this study is: according to local conditions, comprehensive utilization of coal gangue, open up economic and technical feasible way of coal gangue resources.
The mission is:
1. Study the main elements and scattered elements present in coal gangue, and develop economically and technologically feasible and environmentally beneficial extraction processes.
2. Sample the main coal gangue production areas and grasp the distribution laws of the main and rare elements in the coal gangue.
The key technology of this study is to extract the main elements and scattered elements in coal gangue and avoid the problems of low resource utilization, high cost and secondary pollution.
This project will study the comprehensive utilization process of coal gangue: first separate and extract aluminum salt (polyaluminum sulfate) and siliceous (water glass and silica), and leach the coal gangue in the mother solution and enrich gallium. The gallium in the mother solution is enriched with an extractant or a leaching resin, and the gallium is stripped and eluted with an acidic medium. The collection rate of silicon, aluminum and gallium is expected to reach more than 80%.
