Decryption: How to dispose of flying ash into resources?
Recently, the Ministry of Ecology and Environment and the National Development and Reform Commission officially approved the list of the first batch of regional special hazardous waste centralized disposal center projects and their units in the country. Wuhu Conch Environmental Protection Technology Co., Ltd., a wholly-owned subsidiary of Conch Environmental Protection Group Co., Ltd.(hereinafter referred to as Conch Group)(hereinafter referred to as Wuhu Conch) fly ash disposal project (including Chaohu Flying Ash disposal project to be built), is among them.
The selected Wuhu Conch project is positioned as a key category regional disposal center, focusing on the disposal of special categories of hazardous wastes such as fly ash. It is understood that the basic technology used for the disposal of fly ash in the Wuhu Conch Project-fly ash washing and desalination engineering application technology (second generation) is developed by Beijing Zhongke Guorun Environmental Protection Technology Co., Ltd.(hereinafter referred to as Zhongke Guorun).
It is reported that as early as 2019, Conch Group and Beijing Zhongke Guorun reached a cooperation project-Anhui Wuhu Haichuang Fly Ash Resource Disposal Project, which uses fly ash washing and desalination engineering application technology to dispose of waste incineration fly ash. After this technology was selected into the first batch of "Advanced Practical Technologies for Pilot Construction of" Waste-Free Cities "by the Ministry of Ecology and Environment in 2019, Wuhu Conch has once again used this technology to be selected into the centralized disposal of regional special hazardous wastes (fly ash). Center project.
The reporter learned that at present, the engineering application technology of fly ash washing and desalination has been updated from one generation to three generations. So, how does this technology deal with fly ash, and what industry pain points can the third generation technology solve? Our reporter interviewed the person in charge of Zhongke Guorun and relevant experts.
The fly ash is washed and put into the kiln to ensure cement quality
As we all know, fly ash is equivalent to a concentrate of harmful substances in garbage. Although it contains harmful substances such as dioxins and heavy metals, it is still a resource when used properly. Therefore, how to effectively dispose and utilize fly ash is a topic for the industry to explore and overcome.
It is reported that through the collaborative disposal of domestic waste incineration fly ash by cement kilns, dioxins in fly ash can be effectively removed and heavy metals can be solidified in cement. This is another important way to dispose domestic waste incineration fly ash in addition to landfill.
In recent years, many places have introduced policies to encourage the utilization of fly ash resources. The "Three-Year Action Plan for" Zero Landfilling "of Hazardous Wastes in Zhejiang Province" proposes to strengthen the detection and management of pretreatment products such as washed fly ash and refined waste salt.
Among the engineering application technologies of fly ash washing and desalination applied by Wuhu Conch, washing fly ash and evaporating salt making are important steps.
Fly ash contains large amounts of potassium, sodium and chlorine, which affect cement production conditions. In addition, if cement products have high chloride ion content, chloride salts will corrode steel bars and affect building life. Generally, the chloride ion content in fly ash can reach more than 20%, and the proportion of potassium and sodium can reach 15%. These substances can be converted into potassium and sodium salts through the water washing process and turned waste into treasure. The collaborative disposal of waste incineration fly ash to extract salt products in cement kilns is an extension of economic cycle, industrial cycle and industrial chain. This scientific and technological innovation and business model direction is worth promoting. Li Chen, deputy secretary-general of China Cement Association, said in an interview with this reporter.
According to reports, my country has strict regulations on the potassium, sodium and chlorine content of cement. Taking the chlorine content as an example, in the cement quality standard GB175 -2023, the chloride ion content requirement is less than 0.06%(mass ratio). By diluting potassium, sodium and chlorine in fly ash by washing water, the addition ratio of fly ash as cement raw materials can be increased.
So, how is fly ash washed?
According to Li Zhongfeng, chairman of Zhongke Guorun, when waste incineration fly ash is transported to a cement factory qualified for collaborative disposal of hazardous waste in cement kilns through closed tankers, it will be pumped through gas to a closed storage warehouse, and then metered into a certain amount of water, dissolves potassium, sodium and chlorine in the fly ash, and then leaves the fly ash to stand and separates into water and sludge. At this time, the mud has a high water content and still contains some dissolved chlorine, so it needs to be rinsed and diluted again.
It is reported that the fly ash washing and desalination engineering application technology equipment used by Wuhu Conch is equipped with "three-level countercurrent rinsing" equipment. After three washes, the potassium, sodium and chlorine in the sludge are repeatedly diluted, and the content is greatly reduced, which can meet the requirements of cement raw materials.
It is worth noting that because the washing liquor is alkaline, but under alkaline water conditions, dioxins are insoluble in water, and heavy metals are slightly soluble or difficult to dissolve in water. Therefore, after water washing and separation, most of these substances are still left in the fly plaster slag. After the sludge slag enters the cement kiln, it is burned at a high temperature of more than 1000 degrees Celsius, and the dioxins are finally burned and removed, and the heavy metals are solidified into cement clinker according to corresponding standards. At this point, fly plaster residue can be utilized as resources.
Further separate fly ash and reduce dioxin attachment
After the fly plaster slag is disposed of in the cement kiln, it leaves a large amount of water washing solution to dilute the fly ash. These washing liquids are concentrated salt water with extremely high potassium, sodium and chlorine contents. Disposal of these concentrated salt water is also an important part of cement kilns 'collaborative disposal of waste incineration fly ash. This is divided into two steps. The first step is to first solve a small part of the fly ash still remaining in the washing liquid; the second step is to extract potassium, sodium and chlorine.
"The particle size of fly ash is very small, generally ranging from a few microns to tens of microns. Among them, 30 microns accounts for more than 60%. This makes it difficult for fly ash to precipitate easily in the standing pool. On the one hand, if you want to separate fly ash, you must ensure that it has sufficient precipitation time; on the other hand, the particle size is small and the viscosity is very high, and the fly ash can easily hang on the wall., will not easily precipitate to the bottom, which brings difficulties to the extraction of fly ash." Di Dongren, chief engineer of the Research Institute of China Sinoma International Engineering Co., Ltd., director of the Cement Technology and Equipment Professional Committee of the National Building Materials Science and Education Commission, and professor-level senior engineer, told this reporter.
It is reported that at present, one of the commonly used treatment methods for domestic industrial concentrated salt water is to add chemicals for flocculation, and then separate solid substances in the water through precipitation or filtration.
"However, because the particle size of the fly ash is very small, after long-term precipitation and separation by stirring at a higher speed, the fly ash will turn upwards from all directions. Although the wall-hanging phenomenon can be solved, water and fly ash are easy to stir up. In addition, traditional sand filtration or membrane filtration also has problems. The sand filtration process has a backwashing process. The fly ash has a small particle size and will become stuck in the sand and cannot be backwashed. In the membrane filtration process, because the water washing solution contains calcium and magnesium ions and hard fly ash particles, it is easy to pollute and damage the membrane filter layer." Zhao Xiaonan, senior engineer of Zhongke Guorunzheng, said.
In order to solve this problem, Zhongke Guorun has independently developed a set of integrated equipment for slow-settling and stabilization of water washing liquor, which not only achieves an overflow solid content of less than 1%, but a low-flow solid content of more than 20%, which greatly improves the operating efficiency of subsequent processing equipment.
"Specifically, the above-mentioned integrated equipment couples the functions of a radial flow sedimentation tank and a vertical flow sedimentation tank, and distributes water at multiple points to avoid fly ash disturbance. At the same time, the fly ash is given a residence time of about five days for it to settle., then use a mud scraper to scrape out the fly ash at the bottom and put it into the cement kiln again." Li Zhongfeng said that this part is very important because some dioxins are still attached to these fly ash particles in the water. If they are not removed cleanly, it will affect the quality of crystalline salts produced during the evaporation and crystallization process of the next washing solution.
It is understood that the water washing solution after removing fly ash and other impurities is used to evaporate salt production. The condensed water during the evaporation process will be recycled to the front end to wash the ash, which forms a closed loop, and the entire system can achieve zero sewage emissions.
Water washing liquid evaporates to make salt, and the utilization of salt products needs to be explored
After precipitating the water washing liquid and separating the fly ash, the second step of evaporation salt production can be carried out. The main products are sodium chloride salt and potassium chloride salt. This is also the main application scenario of third-generation technology.
Professor He Jie from the Environmental Protection Institute of China General Academy of Building Materials Science said: "The use of water washing technology to dispose fly ash originated in Japan, but Japanese technology does not have the subsequent step of extracting salt from fly ash water washing solution. Generally, heavy metals are discharged directly into the sea after disposal. Therefore, the cost is relatively low."
In fact, the economics of evaporation salt production is a difficult problem facing the industry. Because the calcium content in the fly ash washing liquor is very high, it is easy to form scale during evaporation. Sodium carbonate needs to be added to form a calcium carbonate precipitate to reduce calcium ions in the water. According to reports, the price per ton of sodium carbonate is about 3000 yuan, and the disposal of one ton of fly ash requires about 150kg-180kg of sodium carbonate. Overall, the amount is large and the price is high.
In order to solve the problem of high cost of evaporating salt production, Zhongke Guorun's fly ash washing desalination engineering application technology (third generation) has developed a "double alkali-multi-point dosing" technology, which can ensure the removal of the hardness of the water washing liquid. Based on the effect (<100mg/L), the cost of the drug is greatly reduced.
"The so-called double alkali method is to replace part of sodium carbonate with sodium sulfate. The price of sodium sulfate per ton is more than 500 yuan, which is significantly lower than that of sodium carbonate at 3000 yuan per ton. Adding sodium sulfate and sodium carbonate at the same time in a ratio of 7:3 can not only reduce the hardness of the washing solution, but also reduce the cost of the drug." Li Zhongfeng said.
It is reported that about 25% of sodium chloride salt and about 5% of potassium chloride salt can be extracted per ton of fly ash. Sodium chloride salt can be used as a snow dissolving agent and can also be used in printing and dyeing industry production; potassium chloride is a raw material for agricultural fertilizers. my country's annual consumption of potassium fertilizers is about more than 16 million tons, and the domestic annual production capacity or output is only more than 5 million tons, a large amount of which must be imported (potassium chloride is one of my country's eight scarce mineral resources).
In order to better recover potassium chloride salt, in the fly ash washing and desalination engineering application technology (third generation), a two-stage strong circulation evaporation crystallization process, a segmented crystallization and a digital automatic control system for potassium and sodium salt separation are applied. The recovery rate of potassium salt in fly ash is higher than 80%.
Our reporter learned that recently, the third-generation technology of Zhongke Guorun fly ash washing and desalination engineering application technology used the "double alkali-multi-point dosing" technology, two-stage strong circulation evaporation crystallization process, differentiated crystallization and potassium The digital automatic control system for separation of sodium salts and the integrated equipment for water washing agent slow-settling and stabilizing have passed the appraisal of relevant experts, and the China Building Materials Federation issued them a scientific and technological achievement appraisal certificate.
At present, the third-generation technology has been successfully applied to Hebei Dingxing Guorun fly ash washing resource disposal project. The scientific and technological achievement appraisal certificate shows that the promotion and application of technical achievements will help solve the problems faced by the safe disposal of fly ash in the project location and surrounding areas. Taking the Anshun Taili Zhongrun Project, which also applies third-generation technology, which disposes of 50,000 tons of fly ash per year, as an example. According to full-load operation calculations, about 35,000 tons of limestone raw meal will be replaced every year, saving 1260 tons of standard coal and reducing carbon dioxide emissions of 15,400 tons, saving 10,000 square meters of landfill land, which is in line with the national development plan, and reducing the mining of non-renewable raw materials.
"But it is worth noting that at present, due to safety and other factors, the sales of salt products evaporated by fly ash washing liquid are not good. In the future, how to use these salt products still needs to be explored by the industry." Cui Suping, a member of the Expert Committee of China Building Materials Federation and a professor at Beijing University of Technology, said.
