At the 2024 SMM 19th Lead and Zinc Conference and Lead and Zinc Technology Innovation Forum - Zinc Recycling Industry Salon hosted by SMM, Jin Yonglong, chief expert/doctor/professor of HBIS Group, explained the characteristics, disposal process, improvement research and practice of iron and steel zinc-containing solid waste, and looked forward to the future development trend of the zinc-containing solid waste disposal industry of iron and steel enterprises.
Characteristics and disposal process of iron and steel zinc-containing solid waste
1.1 Characteristics of zinc-containing solid waste
The main source of zinc-containing dust sludge in iron and steel enterprises
Trends in zinc in steel production
The raw fuel is more complex and diverse, and the zinc load into the furnace increases. Many enterprises use iron ore with high zinc content and relatively cheap price, while electric furnace and converter scrap use various types of galvanized or zinc-containing scrap, resulting in a significant increase in the zinc load of blast furnace.
The increase in the scrap ratio, especially the increase in the supply of scrap in society, has reduced the controllability of zinc.
In the process of carbon peak and carbon neutrality, the development of short-process steelmaking is an inevitable trend to achieve green and low-carbon development. As the proportion of electric furnaces increases, the resulting electric furnace ash will increase.
1.2 Zinc-containing solid waste disposal process
A large number of technical research and engineering practices have been carried out at home and abroad for the resource utilization of zinc-containing solid waste, and the main idea is to comprehensively recycle zinc and iron in zinc-containing solid waste, as well as other useful elements, including melt separation (DK blast furnace, OxyCup, etc.), solid separation (rotary kiln, rotary hearth furnace, etc.) and wet separation processes.
The wet process mainly includes acid method and alkali method, which is suitable for dust sludge with high zinc content such as electric furnace ash.
The pyrometallurgical process has low input cost and high acquisition rate of valuable metals, among which rotary kiln and rotary hearth furnace are the two mainstream processes in current industrial production.
Research and practice on improvement of zinc-containing solid waste treatment process
2.1 Selection of disposal process
Energy Consumption Analysis:
Rotary hearth furnace: the total design energy consumption is between 390~430 kgce/t, and the actual production is about 50kgce/t higher than the design value, of which the energy consumption brought in by coal gas or natural gas is between 220~260 kgce/t, accounting for about 59% of the total energy consumption, and the solid carbon is necessary for zinc reduction, the dosage is 10%~12% of the total amount of ingredients, and the carbon consumption of the reducing agent is about 150kgce/t, accounting for about 37% of the total energy consumption, and the rest is the consumption of feng shui and electricity
The total energy consumption of the rotary kiln for the purpose of dezincification and all the reuse of ferrous materials can be comparable to that of the rotary hearth furnace!
Transformation of the operating system:
Traditional rotary kiln:
The main purpose and profit point is to recover the zinc oxide of high zinc solid waste, and the remaining iron-containing part is called "kiln slag", which is separated after fine grinding and magnetic separation, and the tailings with low iron content still have the risk of secondary solid waste; if coupled with the lack of environmental protection and high energy consumption of some enterprises, the society has a poor impression of the rotary kiln to dispose of zinc-containing solid waste, and the promotion in large iron and steel enterprises is limited.
Improved rotary kiln:
For the purpose of removing zinc and recovering iron-containing materials to the greatest extent, the zinc content in the kiln is no longer the main condition for limitation; under the condition of ensuring that the dezincification rate is 90%, the operating system is reasonably adjusted; the premise of batching is that all iron-containing materials can be reused in the steel process without causing secondary pollution.
Improvement direction of traditional rotary kiln:
When selecting the process and operation system for the reuse of zinc-containing solid waste, a high dezincification rate should be the main goal compared with the metallization rate of iron, and the energy consumption should be reduced as much as possible while maintaining high production efficiency.
• Environmental protection standards
• Optimal energy efficiency
• Comprehensive disposal of zinc-containing solid waste in iron and steel enterprises
• The zinc removal rate reaches more than 90%.
•Iron-containing materials meet the requirements of the main steel production process (TFe, Zn content, particle size, etc.), and there is no secondary solid waste pollution
• Investment and production operating costs are better than those of rotary hearth furnaces
• Intelligent and controllable
2.2 Rotary kiln improvement practice - (1) reasonable combination of raw materials
The adaptability of the disposal process is required
The characteristics of blast furnace secondary ash are that it contains a large amount of carbon element, which is an important reducing agent and heat source, and the zinc content of blast furnace secondary ash in some enterprises reaches 8%~10%, and direct recycling is seriously restricted.
The converter ash is characterized by high alkaline oxides (CaO, MgO) and a higher TFe content than the secondary ash of the blast furnace.
The zinc content of electric furnace ash is the highest, and the fluctuation is the largest, some enterprises can reach more than 20%, and even can be sold directly as a lower grade of zinc oxide, but the loss of iron is inevitable.
Reasonable ingredients to ensure heat replenishment
Carbon resources. Considering the full use of carbon resources in zinc-containing solid waste, the use of high calorific value gas can be reduced, and coke powder can be used as a supplementary reducing agent and heating agent.
Reasonable combination of iron resources and zinc content. Not only the recovery of zinc, the high zinc content of zinc oxide can be sold as a product, but another more important purpose is to fully recover iron and achieve "zero solid waste discharge". Therefore, the batching process needs to add some materials with high iron content, so that the iron-rich materials after dezincification will not become an obstacle to the reuse of various processes of iron and steel enterprises, such as grade, strength and particle size and other indicators have different requirements in blast furnaces, converters and electric furnaces. To this end, some enterprises directly add part of the concentrate powder, while some enterprises still focus on high iron content dust removal ash, such as blast furnace tapping yard ash, TFe can reach 56%.
2.2 Improvement practice of rotary kiln - (2) fugitive emission control
Closed or closed transshipment and storage systems
Iron and steel plant dust removal ash and dust removal ash are used suction and discharge tanker pneumatic conveying method into the warehouse, the warehouse is equipped with screw feeder batching, each dust removal ash is dosed feeding, environmental protection effect is good, the batching is uniform.
The use of dry and wet pre-mixing technology can effectively reduce the energy consumption and cost of raw material pretreatment.
2.3 Intelligent and controllable rotary kiln operating system
Hardware: online gas analysis sensors, online temperature sensors, control computers and related consumables
Software: online monitoring system for key data of rotary kiln production
•Kiln temperature measurement and analysis subsystem;
• Kiln tail gas temperature and composition analysis subsystem;
•Early warning event recording and prompting subsystem;
•Data acquisition subsystem;
•Database management subsystem.
Based on the basic concept of the system design, the relevant software has been developed, and the offline commissioning and operation have been realized, which can be installed on the newly designed or modified rotary kiln to realize online monitoring and provide a quantitative basis for the adjustment of operating parameters.
As shown in the figure, the gas temperature and composition analysis subsystem at the kiln tail is in operation. The subsystem can effectively reflect the changes in the production status of the furnace under different conditions, and at the same time, according to the corresponding event prompts and solutions given by the system, the production parameters can be adjusted in a targeted manner to achieve the best production state.
Effect:
(1) Reduce the risk of falling or collapsing of the knots bound to the kiln wall, and correspondingly reduce the possibility of damage to the kiln wall; reduce the probability of chemical reaction between the substances in the knots and refractories at high temperatures, and improve the life of the refractories; reduce the phenomenon that the formation of the knots hinders the movement of materials and air flow in the kiln, resulting in the stagnation and accumulation of materials in the rotary kiln.
(2) Reduce the phenomenon that the movement of materials and coal gas flow is hindered by the generation of knots in the rotary kiln, so that the materials cannot be calcined completely in the kiln, reduce the quality of iron-containing secondary resources, and improve the quality of finished products and the production efficiency of the rotary kiln.
(3) Improve energy efficiency. If the ring of the rotary kiln is serious, the movement of the material will be seriously hindered and the normal production will not be possible, at this time, the kiln must be stopped to deal with the ring material in the kiln, and the circle object can be re-opened, ignited, heated and fed after the ring material is thoroughly cleaned up. If production cannot be carried out due to frequent looping, energy will be wasted.
(4) The operation process of the rotary kiln does not require additional high-order gas with high calorific value, and the temperature area control requirements in the furnace can be achieved by using its own carbon distribution, co-disposal of oil-containing or carbon-containing sludge and CO generated by reduction, or a small amount of low-rank gas heating.
2.4 Fully quantified resource recovery and purification system
Recycling of iron-containing materials
Closed design of kiln head. The kiln head of the rotary kiln is equipped with a secondary resource collection device for iron content after reduction and dezincification. In order to avoid the generation of water vapor (cooling water evaporation) and dust, a special kiln head seal design is used.
Purification system
The flue gas recovers heat through the waste heat boiler (select the setting) or other waste heat devices, and is discharged after the flue gas desulfurization (denitrification can be selected according to the actual raw fuel conditions) after bagging;
Recycling of zinc products
The flue gas at the end of the rotary kiln passes through the oxidation sedimentation chamber to settle the large particles of dust, and returns to the rotary kiln through a special lifting system; the waste heat device and the lower part of the cloth bag are provided with a device for recovering zinc oxide, which is collected and sold according to the zinc oxide content.
2.5 Application effect of improved rotary kiln
On the basis of a large number of investigations and experimental studies, drawing on the successful experience of rotary kilns specializing in the disposal of zinc-containing solid waste in iron and steel enterprises such as Benxi Iron and Steel and Nanjing Iron and Steel Corporation, the advantages and disadvantages of rotary kilns and rotary hearth furnaces are fully compared, and the rotary kiln system is constructed by using the factory buildings and vacant land reserved for the production of iron scale tunnel kilns in combination with the actual situation of Wugang of HBIS Group.
The technical equipment of the improved rotary kiln process production line has been put into operation in Hegang Wugang Industrial Company in October 2019, and after many adjustments and optimizations, the rotary kiln has been running stably so far, and it is only repaired once a year. The zinc content of the furnace is about 4%, the carbon blend is about 15%, the dezincification rate of zinc-containing materials is 92%, the TFe of iron-containing secondary resources is 62%, and the zinc content is about 0.3%.
It is a good solution to the problem of the difficulty of zinc-containing solid waste disposal in iron and steel enterprises, and can realize the efficient recovery of zinc-containing products and iron-rich secondary resources, with simple process, low operating cost, high-efficiency dezincification, and no secondary solid waste pollution.
Reflection and outlook
1. The future trend of zinc-containing solid waste disposal in iron and steel enterprises is: low energy consumption > high energy efficiency, environmental protection standards, simple process, low operating cost and low investment cost;
2. The improved rotary kiln can also be used as a co-treatment of solid or hazardous wastes such as oily sludge and municipal fly ash in enterprises, making full use of the combustible substances in them, but it is necessary to design supplementary facilities such as secondary combustion subsystems to control dioxine emissions.