The implementation of a treatment process needs three scientific sections as support: first, correct process conception; Second, the equipment production line is wide enough to accommodate the process ideas; Third, there is a safe and stable operating system that can accommodate the process concept and equipment operation.

Li Chaoxia, Chairman of Luoyang Luohua Powder Engineering Special Refractory Materials Co., Ltd

Huanghe, General Manager of Luoyang Luohua Powder Engineering Special Refractory Materials Co., Ltd

Li Hongtao

Red mud is a highly alkaline waste produced in the production process of alumina industry. Its composition is complex and its comprehensive utilization is difficult. Up to now, there is no cost-effective large-scale disposal and comprehensive utilization technology, resulting in a global utilization rate of less than 10%. It is expected to become the next profit outlet for alumina production by challenging the world's problems, "eating dry and squeezing clean" red mud, and realizing resource utilization.

Current situation

Red mud is a kind of solid waste residue formed in the alumina production process when bauxite is leached in strong alkali due to iron, titanium and other impurities in the ore and most of silicon are insoluble. It is named because it is mostly red or brown. As the fourth largest alumina producer in the world, China's production of red mud has increased year by year due to the low grade of bauxite used in recent years. According to statistics, according to the existing domestic production methods and technical levels, 1 ton to 2.5 tons of red mud can be produced for each ton of aluminum oxide produced. In 2019, China's alumina output will be about 72.474 million tons. If 1.5 tons of red mud are produced for each ton of alumina produced, the annual discharge of red mud can reach about 100 million tons. According to the analysis of the Development Report on Comprehensive Utilization of China's Bulk Industrial Solid Wastes from 2019 to 2020, the comprehensive utilization of red mud in China will be 8.49 million tons in 2020, with a comprehensive utilization rate of only 8%, less than 10%. This is far from the policy goal of "increasing the comprehensive utilization rate of red mud to 10% by 2020" proposed during the "13th Five Year Plan" period. With the increasing output of red mud and people's constant attention to environmental problems, it is urgent to make comprehensive use of red mud to a large extent and limit its harm. This is also an important task to be solved during the "Fourteenth Five Year Plan" period to promote green industrial development. Under the new situation, with the change of laws and regulations, policies, technologies, environment and other conditions, the comprehensive utilization of red mud is in the critical period of breaking through large-scale utilization. However, in China, the reality is that most of the red mud treatment methods still stay at the low and shallow storage level of stacking in the yard, dam building wet stockpiling or dry stockpiling of red mud after drying and dehydration. The stockpiling of red mud not only occupies a lot of land and farmland, but also the dust formed by dried red mud is flying everywhere. These methods not only destroy the ecological environment, but also make many available components in red mud not be used reasonably. How to realize the resource utilization of red mud to ensure environmental protection and create greater benefits has become the focus of the government and enterprises.  

Dilemma

Red mud can be divided into Bayer red mud, sintering red mud and mixed red mud according to different production methods in alumina production. The former is mostly used when the aluminum content in bauxite is high, while the latter two are used when the aluminum content in bauxite is low. Although the composition of these three types of red mud is different, one thing is the same - they are all 0 μ m~10 μ M, and the viscosity is strong due to long-term immersion precipitation. In the post treatment process, if the method is improper, the red mud micro powder agglomerates are broken blindly, which is more likely to cause dust explosion and become "red mud dust haze" for large-scale diffusion. However, the "red mud dust haze" can not bear and absorb the additional energy, and can not be effectively collected, thus losing the conditions for physical transformation, resulting in treatment failure. In addition, because the chemical alkali combined with red mud is difficult to remove and has a large content, if the method of gradual decomposition in multiple process sections is used to remove the alkali first and then separate and transform the red mud, the production line is complex, the equipment volume is huge, the thermal efficiency is low and the cost is high, and the solution is very weak for the red mud that is increasing year by year and piled up like a mountain. Moreover, due to the strong alkalinity of red mud, its comprehensive utilization is far more difficult than other industrial wastes. Xu Delong, an academician of the CAE Member, said publicly in an interview with the media: "The red mud produced by alumina and other factories generally contains aluminum, iron and other heavy metal salts. It can effectively recover aluminum, iron and other substances from red mud, but it is technically difficult to achieve pollution-free recovery." Faced with the dilemma of "large amount of red mud is useless" and there is no place to place it, Relevant domestic departments and some enterprises have also put forward and implemented specific treatment methods, but later practice has proved that most of them address the symptoms rather than the root causes. For example, "vegetation improvement method" incorporating homogenization. This method adds a small amount of red mud to a large amount of ordinary clay, without physical decomposition or synthesis, and directly lays it on the ground to plant plants. It is called "soil vegetation transformation", which is actually "artificial saline alkali land". Another example is "building filling method" with homogenization. In the process, a small amount of red mud is added to the building concrete, ash brick and building ceramsite to replace some cement, and the heavy alkali water glass material of red mud is used to replace the silicon gel in some cement, which is actually a "red mud moving" practice. Because the calcium silicate gel in cement comes from natural minerals and has a stable organizational structure, it is a "hydraulic gel", while red mud is an unstable "thermosetting fine powder agglomeration" immersed in aluminate gel and compound calcium silicate gel. When added to concrete structures at room temperature, it will always exist in an unstable state, causing a series of serious derivative consequences. Another example is the iron reduction method of red mud physical selection. The high iron red mud is filtered to remove part of the liquid, and then the ferromagnetic substances in the red mud are magnetic separated. This is a comprehensive utilization of red mud, which consumes energy but is not "cost-effective". Because, although some ferromagnetic substances have been removed, the total amount of harmful substances in red mud has not been reduced. The bulk density of the remaining red mud is smaller and the alkali concentration is greater, making the dry red mud closer to the hazardous waste standard of combustible explosives. The domestic red mud treatment method is still not feasible, and it is difficult to realize the harmless utilization of red mud internationally. Experts all over the world have carried out a lot of scientific research on the comprehensive utilization of red mud, but little progress has been made at present. The red mud is still mainly disposed by stacking on the sea floor or on land. However, with the development of aluminum industry, the amount of red mud discharged from alumina production is increasing day by day, and a series of problems caused by stockpiling and disposal follow. In China, the average storage cost per ton of red mud is about 50 yuan per year. Considering the total amount of tens of millions of tons of red mud produced each year, it can be regarded as a huge capital consumption. Other losses, such as pollution to the environment and damage to the human body, are even more difficult to count. In the face of this unavoidable worldwide problem in the development process of the alumina industry, it still requires a lot of efforts from all parties to achieve harmless treatment, especially the breakthrough of key technical difficulties. Based on the principle of "systematization and zero emission", research and develop new technologies for joint recovery, and optimize the existing processes, so as to realize the reduction, recycling and harmlessness of comprehensive recovery and utilization of red mud.

Large rotary kiln with high load and stable operation for more than five years

Conception

Based on years of scientific and technological development experience, Li Chaoxia, chairman of Luoyang Luohua Powder Engineering Special Refractory Co., Ltd., believes that the implementation of a treatment process needs three scientific sections as support: first, a correct process concept; Second, the equipment production line is wide enough to accommodate the process ideas; Third, there is a safe and stable operating system that can accommodate the process concept and equipment operation. In terms of the process concept of red mud treatment, in view of the emergency situation that China has accumulated a large amount of discharged red mud on the ground over the years, and the annual amount of red mud continues to increase by tens of millions of tons, at this stage, it is urgent for China to carry out the development and research of the production line of "industrialization of red mud resource treatment", so as to protect the living environment of China and prevent problems before they occur. Therefore, on the basis of the process concept, Li Chaoxia specially applied to the relevant departments for the invention patent of "Rotary Kiln Process for Resourceful Treatment of Red Mud and Rotary Kiln". The purpose of this patent is to "transform red mud into resource materials that can enter the industrial and civil markets immediately at one time". Its process concept can be roughly summarized as follows: the red mud treatment process is a short process powder chemical process production line with rotary kiln as the main equipment, which can utilize multiple energy sources, and can be explosion-proof and energy-saving. Specifically, it refers to adding carbon containing materials and other additives into the red mud containing sodium (calcium) alkali solution, taking a rotary kiln as the main equipment for process treatment, and adding the mixed materials of red mud containing sodium ferrite heavy alkali water glass solution, sodium aluminate mixed solution, carbon and other additives into the rotary kiln. Through the metallurgical reaction in the rotary kiln, the red mud is separated, decomposed, reduced or synthesized in a different way from the conventional metallurgical process. Metals such as ferromagnetic materials, industrial colored glasses, and sodium (calcium) metal salt ultrafine powder and high-purity gaseous silica ultrafine powder are extracted in turn. Different from the traditional treatment process, the energy used in this process includes the hydration energy and chemical erosion energy of the heavy alkali liquid to the red mud, the kinetic energy of the heavy alkali liquid to disperse and gather the mixed materials when boiling at medium and low temperatures, the water gas erosion energy of the carbonaceous substances to the red mud under the catalysis of sodium (calcium) salt environment, and the heat energy released when carbon monoxide is generated and converted to carbon dioxide, And the strong reduction impact on red mud to break the thermal dynamic force. In the process of implementation of the patented process, the strict requirements of "double fast and double high" change of materials and temperature must also be met. "Double quick" means that once the mixed materials are added into the rotary kiln, the physical and chemical properties of the materials and the speed of temperature rise will change rapidly; "Double high" includes two aspects. One is that the high temperature process of oxygen enriched converting of rotary kiln makes use of the alkaline hydration energy of red mud and the trend energy of self recovery stability of red mud, which is a high energy saving process. The other is that the conversion rate of dealkalination of red mud with unstable structure to stable substances is high. The requirements of "double speed and double height" make the process face various severe tests in the operation process. For example, red mud is a highly alkaline and highly active micro powder agglomeration material, and its composition is diverse, which requires flexible and diverse ingredients system; There are many energy nodes with intense reaction in red mud recycling heat process, and technicians must strictly grasp the nodes to ensure production safety and stability; The specific surface area value of red mud particles is very high, so special protective measures should be taken in the process to prevent the breaking of micro powder agglomerates without simultaneously entering the stable transformation of material molecular structure, so as to eliminate alkaline dust haze; When treating activated red mud, the mutual adaptability of process requirements and process equipment must be considered, which must be included in the consideration of process R&D personnel and enterprises.  

Equip

In the process of treating industrial solid wastes in China's environmental protection industry, large rotary kilns have become one of the mainstream equipment for treating industrial solid wastes and hazardous wastes with their remarkable performance of environmental protection, energy saving, easy operation, etc. The ballad "As long as the big kiln turns, there will be tens of millions" is a vivid description of the importance of the rotary kiln in the solid waste treatment process. Rotary kiln, also known as rotary calcining kiln, is commonly known as rotary kiln. In the construction process, it carries out mechanical, physical or chemical treatment of solid materials through rotary cylinder equipment, which has been widely used in building materials, chemical industry, environmental protection and many other production fields. In terms of red mud treatment, the metallurgical rotary kiln equipment is one of the ideal equipment nowadays with its excellent performance—— The equipment is large in size and has strong material tolerance. Whether it is slurry like materials or block like granular materials, whether it is granular or powdery materials, it can be treated by rotary kiln, and various and flexible feeding forms can be used according to the characteristics of materials—— Compared with the materials in the kiln, the working volume of the rotary kiln can be regarded as infinity, which can meet the process requirements of large-scale and large processing capacity—— With the help of the mechanical rotation mode of the rotary kiln, the materials entering the kiln obtain their own kinetic energy and potential energy. Under the condition of keeping the materials in a relatively intense collision, tumbling and friction motion, it can promote the transformation of material and mud in the hot state and reduce the requirements of red mud in the rotary kiln on other metallurgical energy. This is also a unique energy-saving mode of rotary kiln—— Rotary kiln equipment can conduct "fine work and slow fire" heating and smelting of materials. The materials entering the kiln can gradually increase the accumulated value of metallurgical field strength, so as to improve the metallurgical efficiency and reduce the energy cost—— In the rotary kiln, materials are added from the kiln tail and run against the generated hot flue gas to stimulate the metallurgical reaction of materials so as to obtain higher metallurgical energy level. This energy-saving form of directly using hot flue gas to improve the "positive feedback" thermal effect of metallurgical process is also unique to the rotary kiln metallurgical process—— The rotary kiln has the characteristics of elastic semi closed working chamber, and has the ability to self recover emergencies, prevent fire and explosion. It is possible to directly feed the activated materials and materials containing liquid substances into the kiln for treatment. In addition, the rotary kiln equipment has large volume, relatively small manufacturing difficulty, large output and easy operation, so it is easy to be accepted by investors. The use of large rotary kilns has thus become one of the important equipment in the metallurgical industry. However, because the quality and service life of the traditional rotary kiln lining can not meet the needs of industry, people can only put the rotary kiln in the low-end material processing process at this stage, which can not fully show its many advantages such as energy saving, large output and strong adaptability in the form of metallurgical equipment. To this end, Li Chaoxia and his company specially invented the series of "Long life large rotary kiln castable integral lining technology". Compared with the old rotary kiln lining technology, this technology series has carried out subversive transformation and innovation for the furnace lining structure and lining materials, which makes the products not only energy saving and environmental protection, but also have significant production expansion, improved equipment operation efficiency, long operation time and other superior performance. As for the treatment of such materials as red mud, the excellent furnace lining technology can further ensure that when red mud enters the rotary kiln and its temperature is raised by oxygen enriched blowing, the separation of red mud can be accelerated, so that the metal reduction, non-metallic oxidation and metal salt synthesis of red mud can be carried out simultaneously, and the red mud can be rapidly decomposed into three types of materials: metal ferromagnets, non-metallic oxide glass, metal salts and alkaline silicide superfine powder. At the same time, using the mechanical energy of rotary operation of the rotary kiln, the red mud mixture can be continuously and repeatedly cross homogenized with high efficiency, so as to improve the metallurgical efficiency, accelerate the concentration of similar substances and the separation speed of different substances, and separate them at the outlet of the rotary kiln head according to the physical characteristics of different materials after conversion and discharge them out of the furnace. Therefore, Li Chaoxia believes that on the basis of the scientific and technological achievements of "long-life large-scale rotary kiln composite lining", we can learn from the experience of the process and equipment route of Reductive Smelting Smelting Process of Laterite Nickel Ore (Patent No.: ZL201910492256.1) and Castable Lining and Zinc Leaching Slag Rotary Kiln with this Lining (Patent No.: ZL201910477732.2) that have been continuously developed and put into practice for many years, As well as the patent line of the Invented Rotary Kiln Process for Resourceful Treatment of Red Mud and Rotary Kiln (211149AP under review), we have made precise equipment configuration for red mud treatment, so as to further carry out research, development and implementation of red mud industrialization.  

Operation

In terms of the operating system that can accommodate the process concept and equipment operation, Li Chaoxia put forward several ideas and suggestions of his own, so that the later operation work can be carried out stably and effectively. With the help of the existing advanced and perfect operating system and experience of large-scale building materials rotary kiln in China, the basic process flow is formulated by combining the patent ideas and achievements of Luoyang Luohua Powder Engineering Special Refractory Co., Ltd., the experience and data accumulated in the rotary kiln lining material technology and construction technology, and the alumina production process and standard, as well as the cement production process and standard. Second, the existing cement and building materials rotary kiln production line should be improved and transformed to meet the needs of the production line of "rotary kiln method for resource treatment of red mud". Third, the industrial process test shall be carried out first. After the process test and process route correction, various process data and correction methods shall be stored in the computer, and the computer shall be used to track, correct and improve the real-time process. As the second bulk of solid waste materials to be treated in China's metallurgical industry, second only to steel slag in the iron and steel industry, red mud is piled up on the ground in the form of high alkali micro powder agglomerates all year round, which causes great harm to the environment and high storage cost. However, everything has two sides. Since the mother body of red mud is high quality bauxite with high stability, the reverse synthesis principle of its main components directly points to the industrial necessary metal raw materials such as heavy metal ferromagnets, clay non-metallic oxides, sodium salts and other metal compounds, and the chemical composition is unified with large output, so it is suitable for industrial layout. We also believe that in the near future, the resource utilization of red mud will be full of vitality, thus opening up a new green environmental protection industry. Red mud conversion products will also become one of the treasure houses of new resources, adding luster to the cause of green water and blue sky in China. (The drawing is provided by Luoyang Luohua Powder Special Refractory Co., Ltd.)

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