Comprehensive utilization of coal ash

Due to extensive research both domestically and internationally, its current utility value is low; it is only used in the production of cement or bricks, and in some cases it cannot even be used for that purpose (due to excessively high levels of heavy metals).

Currently, the high-alumina fly ash from the Datang Tuoketuo Power Plant is being used to extract alumina, and research is being conducted on the electrolytic production of aluminum ingots from this alumina. If successful, this project will bring significant economic, environmental, and social benefits to the recycling of fly ash.

If the content is high, there is still value and potential for recovery.

Coal from different regions varies in the composition of its ash, and many people are currently conducting research on magnetic separation of ash to recover iron. If it is feasible, it is also highly worth promoting and emulating

Flotation beads can be selected from coal ash, and it can also be used to make building materials such as bricks, blocks, and cement!

We use the ash from coal-water slurry gasification units to manufacture building bricks, and the results are quite good! Further processing is required before it can be used as raw material for manufacturing bricks

The content is too low, so there are some issues with recovery

High heavy metal content means low utilization value?

After burning in a boiler, coal leaves two solid residues: ash and slag. The solid particles that are discharged with the flue gas from the rear of the boiler, and which are mainly collected by dust collectors, are fly ash ； Particles that are larger or come in lumpy forms, and are collected from the bottom of the furnace, are known as bottom slag. From the perspective of comprehensive utilization, fly ash generally also includes slag, which is a general term for ash and slag. Fly ash is mainly composed of silico-aluminosilicate glass, microcrystalline mineral particles, and unburned carbon residue particles, with silicon oxide and aluminum oxide being its primary chemical components. Most of the fly ash in our country comes from coal-fired power generation boilers in large and medium-sized thermal power plants, while another portion comes from fly ash boilers used for centralized urban heating. At present, fly ash is mostly discharged in a wet form, which requires a large amount of water ； Stacking requires a large amount of land. In 1999, China’s fly ash emissions reached 160 million tons. According to statistical analyses and projections, based on the current levels of ash emission and utilization, the water required for dealing with this ash amounts to over 1 billion tons per year ； The ash storage area covers approximately 500,000 mu, with the cumulative amount of ash stored over the years exceeding 1 billion tons. Although the amount of ash utilized each year is increasing, the overall utilization rate is still less than 50% of the annual emissions. As the installed capacity of the power industry increases, the amount of ash discharged, water consumption, and land required also increase accordingly. At the same time, wet ash disposal not only consumes water and electricity and pollutes the environment, but it also reduces the activity of fly ash, which hinders its comprehensive utilization. As China’s technology for dust removal and dry ash transportation continues to improve, power plants should make use of efficient dust collectors in the future, and design separate systems for collecting dry ash, so as to enable greater utilization of fly ash. For the fly ash collected by wet dust collectors, dehydration devices should be installed or the fly ash should be dried out to reduce its moisture content to below 30%, thereby creating conditions for the comprehensive utilization of the fly ash. Fly ash is piled up in the open; on windy days, dust contaminates the air, and on rainy days, water seepage contaminates groundwater. According to domestic and international experimental studies, the infiltration of fly ash causes varying degrees of pollution to groundwater; notably, it leads to an increase in pH levels as well as higher concentrations of toxic and harmful elements such as chromium and arsenic. Furthermore, since most fly ash storage sites are located in areas surrounding rivers, lakes, and urban water sources, the issue of water source protection is also extremely urgent. A brief overview of the comprehensive utilization technologies for fly ash in China. Fly ash as a building material: Fly ash was first used in China for the production of building materials, with an utilization rate that has remained around 25% throughout. Fly ash-sintered bricks, the production of cement clinker and its use as an admixture, the production of ceramsite, blocks, aerated concrete, wall materials, etc., are all **established technologies that are being promoted. In 1998, the total amount of wall materials in China was equivalent to 860 billion standard bricks, of which over 700 billion were solid fired clay bricks. 1. Production of sintered bricks using fly ash: The proportion of fly ash used ranges from 30% to 70%, and the main processes and equipment are essentially the same as those for ordinary clay bricks. A factory in Jilin that produces sintered bricks from fly ash uses the wet-fly ash discharged by the Jilin Thermal Power Plant, which is naturally dehydrated to a moisture content of around 30%, and formulates the mixture using 55% fly ash, 40% clay, and 5% industrial waste such as slag. The plant uses 400,000 cubic meters of fly ash per year, produces 240 million fly ash-sintered bricks, saves 430 cubic kilometers of clay annually, and reduces coal consumption by 9,600 tons per year, thereby delivering significant social and economic benefits. 2. Production of steam-cured bricks from fly ash (referred to as steam-cured bricks). The ingredients used in the production of fly ash steam-cured bricks include fly ash, which can account for around 65% of the mixture; in addition, an appropriate amount of aggregate, quicklime, and gypsum are also required. After the raw material is prepared and shaped by pressing, it is fired into bricks after being cured under normal or high-pressure steam. Its requirement for fly ash is that the lower the carbon content in the ash, the better, and the higher the activity of the ash, the better. 3. Production of non-baked, non-steamed bricks from fly ash: In order to turn fly ash from a source of problem into a valuable resource, Jiangxi Guixi Power Plant has developed new types of fly ash bricks that do not require baking or steaming and can be cured at low temperatures. Its main ingredients are: 70% fly ash, 15% bottom slag, and 15% quicklime (as an activator). The product can reach the grade of No. 75 fly ash bricks; the total amount of ash used in production is 85%. With an annual production of 10 million bricks, 20,000 tons of ash can be utilized, generating an annual profit of 500,000 yuan while saving 300,000 yuan in costs related to ash disposal. It saves 400,000 to 500,000 yuan in construction costs for ash disposal sites, reduces the occupation of arable land by 130 m2, and offers good environmental and economic benefits. 4. Production of silicate blocks from fly ash: Silicate blocks made from fly ash are produced using fly ash, lime, gypsum, and binders as raw materials; in the mixture, besides slag which accounts for about 55%, fly ash can also make up up to 30% of the composition. It is formed by mixing with water, vibrating, and then curing with steam. The requirement for the quality of fly ash in this process is that its loss on ignition be below 15%. It is suitable for industrial and residential buildings; it has better thermal insulation properties than clay bricks, is lightweight, and can meet the fire resistance requirements for load-bearing walls in ordinary buildings. 5. Foam glass made from fly ash: Foam glass is a new type of building material that can be produced by firing fly ash – which can account for 70% of the ingredients – as the main raw material; its density ranges from 0.5 to 0.8 t/m3. It possesses properties such as compressive strength, heat insulation, sound insulation, water resistance, and the ability to float on the surface, making it an excellent material for modern high-rise buildings. Foam glass, used as a material for large-scale sculptures, can be made into large pieces that can be cut and assembled as desired. Wall bricks made of foam glass have a density that is only 5% to 10% of that of ordinary clay bricks, yet their strength is 8 to 15 times higher; therefore, they offer advantages such as light weight, high strength, and energy efficiency. Using it as a material for insulation, heat isolation, and soundproofing offers the advantage of being cost-effective, resulting in high economic and social benefits. 6. Production of aerated concrete from fly ash: Aerated concrete is manufactured using fly ash as the main raw material, along with appropriate amounts of cement, gypsum, aluminum powder, and other additives, to create a lightweight concrete; fly ash can account for around 70% of the total composition. In 1998 in Shanghai, nearly 540,000 tons of fly ash were used solely for concrete, accounting for 15.6% of the total amount of fly ash utilized. A factory in Beijing uses dry discharged coal ash from Gaojing Power Plant as raw material to produce 200 km3 of aerated concrete products per year. It is mainly used for roof insulation, interior and exterior walls, and balcony partitions. It has good social and economic benefits. 7. Production of ceramsite from fly ash: Using fly ash as the main raw material, along with a certain amount of binder and water, and through balling and sintering, the resulting lightweight aggregate is known as sintered fly ash ceramsite. It is a high-performing artificial light aggregate, with coal ash accounting for about 80% of its composition. Concrete grade 300 can be prepared. A factory in Tianjin uses wet-coaled ash from Tianjin No.1 Power Plant as raw material to produce 90,000 cubic meters of fly ash ceramsite annually. Due to its advantages such as low density, high heat resistance, good resistance to contamination, and strong impact resistance, it can replace natural aggregates in the production of concrete with grades 150–300. It is widely used in industrial and civil construction for manufacturing various concrete components, as well as in the construction of bridges, furnaces, and chimneys. For example, in the road deck of the Nanjing Yangtze River Bridge, the use of fly ash ceramsite to produce ceramsite concrete with a grade of 250–300 reduced the bridge’s weight. 8. Fly ash can be used in mortar to replace part of the cement, lime, or sand. Mortar is used in large quantities in construction projects; although the quality requirements for fly ash are not very high, it can improve the properties of concrete and help save cement. This technology allows for the extensive use of fly ash; 50–100 kg of fly ash can be used per cubic meter of concrete, thereby saving 50–100 kg of cement. High-quality fly ash is widely used in the Three Gorges Project, with an annual consumption of nearly 300,000 tons; this has set world records for annual casting volume and maximum casting strength. The proportion of ash used in this technology is over 10%. 9. Fly ash can be used as a substitute for clay as a raw material in cement production. Since its chemical composition is similar to that of clay, fly ash can replace clay in cement manufacturing. Its production process and technical equipment are the same as those for producing ordinary Portland cement. The Shenyang Cement Plant uses the wet-slurry fly ash from the Shenyang Thermal Power Plant as a raw material to produce 120,000 tons of pozzolanic silicate cement per year. 10. Fly ash as an admixture in cement production: When high-quality fly ash is used as an admixture in the production of cement, it is possible to produce ordinary Portland cement, slag Portland cement (with a maximum inclusion level of 15%), fly ash cement (with inclusion levels ranging from 20% to 40%), and low-grade building cement with an inclusion level of 60% to 70%. A building materials factory in Texas utilizes the dry-discharged fly ash from the Texas power plant to produce over 150,000 tons of Portland cement per year, having manufactured both 325 and 425 grade R-type fly ash Portland cement. It has achieved economic benefits of over 700,000 yuan in annual profit, as well as good environmental benefits. The Yancheng Cement Factory in Jiangsu Province uses about 25% dry fly ash from the Yancheng Power Plant to produce 425-grade fly ash silicate cement, achieving annual economic profits of over 1 million yuan as well as significant social benefits. Fly ash is used for underground backfilling and filling of mine subsidence areas. Mine No. 11 of Pingdingshan Mining Bureau, in order to extract steeply inclined and very steeply inclined coal seams, successfully experimented with using fly ash from the nearby Yaoyu Power Plant as a material for underground grouting for fire prevention and filling purposes. Using fly ash grouting to fill the goafs can achieve fire prevention effects, and it can also significantly reduce surface displacement. After filling the goafs with fly ash, it strengthens the surrounding rock and coal pillars, enhancing the strength of these pillars and facilitating the maintenance of roadways. It is also beneficial for the stratified mining of thick coal seams, improving coal recovery rates. The Huaibei Mining Bureau uses the fly ash from the Huaibei Power Plant to fill the subsidence areas in coal mines such as Xiangcheng, Zhuzhuang, Zhangzhuang, and Daihe. This mining area adds more than 2 Mm3 of land subsidence area each year. To alleviate the strain on the ash disposal site at the Huaibei Power Plant, the idea of constructing an experimental ash disposal site in the subsidence area was put forward in 1979. Located just 4 km away from the power plant, this site covers an area of 280 km2 and can hold 700 km3 of fly ash. Ash dumping began in February 1980; after the site reached the designed elevation, soil was applied to transform 432 mu of land into arable fields, resulting in good social and economic benefits. In 1997, 650,000 tons of ash were used for backfilling in Anhui Province, accounting for 26% of the total ash used that year. The proportion of ash used for land reclamation by filling is around 25%. Fly ash is used in road construction, as a component of the road surface base layer. Replacing traditional gravel with fly ash-lime stone (lime, fly ash, and gravel) can **improve the quality of roads, speed up construction, and reduce costs. Today, fly ash is widely used in the embankments of high-grade highways; it offers better quality than binders and also saves a significant amount of land. In recent years, the proportion of ash used in road construction has risen rapidly, reaching around 30%. Just the Shanghai-Nanjing Expressway uses 7 million tons of fly ash. Extracting various chemical and industrial raw materials from fly ash 1. Alumina extraction by alkaline method: Many types of coal ash contain 25% to 40% or more Al2O3, so extracting alumina from fly ash is both economically beneficial and environmentally advantageous. Countries such as Poland use high-temperature sintering (above 1300°C) to extract aluminum hydroxide by subtraction, while in China, processes involving autoclaving at normal pressure and dehydration at low temperatures (around 900°C) are more commonly studied, as they offer lower energy consumption. 2. Recovery of iron or magnetic beads: The iron in fly ash exists mainly in the forms of Fe2O3, Fe3O4, and iron silicate. Iron recycling generally uses magnetic separation. It can first be pre-selected using a cyclone, and then separated with a weak magnetic separator to enrich and obtain high-grade iron ore concentrate. For example, the iron content in the fly ash from Shandong Xinwen Power Plant was 7.68%; after enrichment and mineral processing, the grade of the concentrate increased to 55.08%, with an iron recovery rate of 47.90%. 3. Recovery of hollow microspheres: Hollow microspheres are formed when coal is burned at high temperatures of 1350–1500°C, reaching a molten state; after being atomized by high-pressure air currents, they coalesce into microspheres due to their own surface tension. These are hollow spherical particles that are produced when the ash cools down during disposal. Their particle size generally ranges from 0.25 to 150 microns, with some reaching 300 microns. Based on the thickness of the bead wall, they are further divided into floating beads and sinking beads. Flotation beads can be used for flotation purification. For example, Nantong Tianshengang Power Plant uses this method to select beads with a loss on ignition of 0.6%, accounting for about 80% of them; the cost is only over 10 yuan per ton, while the value can be increased by 60–80 yuan per ton. Sedimented beads can be recovered using gravity separation. By using a hydrocyclone for one or multiple open-circuit classifications, and taking the fly ash from Power Plant 502 in Panzhihua as the raw material, three-stage open-circuit classification with a hydrocyclone was employed, resulting in a separation efficiency with a sediment bead content of 85%~95% and a recovery rate of over 75%. Hollow microspheres possess a variety of excellent properties such as spherical shape, small size, light weight, hollow structure, high temperature resistance, electrical insulation, and high strength. They can be widely used in industries such as refractory materials, plastics, rubber, petroleum, electronics, aviation, submarines, and military applications. Furthermore, due to its large specific surface area, strong adsorption capacity, and the property of polymerization, fly ash can easily adsorb, reduce, and concentrate certain rare elements such as germanium and gallium; there are mature processes and experience available both domestically and internationally for this purpose. Currently, the amount of ash used for extracting chemical and industrial raw materials accounts for 2% to 3% of the total ash volume. Production of magnetic compound fertilizers using fly ash: The Power Generation Branch of the Coal, Electricity, and Coking Plant of Zixing Mining Bureau developed a product for manufacturing magnetic compound fertilizers from the fly ash produced by that plant. The magnetic properties inherent in these fertilizers stimulate crop growth, activate the soil, and enhance the roots’ ability to absorb nutrients from the soil. Fertilizers contain a complete range of nutrients; in addition to the three main nutrients N, P, and K, they also include trace elements and essential nutrients required by crops such as Si, Fe, Al, Mg, Ca, B, Zn, Mn, and Cu. This compound fertilizer has been widely used in demonstration trials on agricultural and fruit crops such as rice, tea, tobacco, and oranges, and it has been proven to significantly increase yields for these crops. The proportion of ash used in this regard is around 10%.

May I ask whether slag with a carbon content of 5-8% can be used as raw material for cement plants? We plan to use the Lurgi gasification process, and the slag produced has a relatively high carbon content.