Technology--Key influencing factors and process technology optimization of prebaked anode quality (Part 1)

As the "heart" of the aluminum electrolytic cell, the prebaked anode is not only the conductive electrode of the aluminum electrolytic cell, but also participates in the electrochemical reaction of the aluminum electrolytic cell. It can be said to be one of the most important components in the aluminum electrolytic process.

In recent years, with the popularization and application of super-large aluminum electrolytic cell technology, it also puts forward higher requirements for anode quality. How to produce high-quality pre-baked anodes with uniform indicators is very important for carbon enterprises. This paper focuses on the influencing factors of anode quality, and puts forward some ideas for the optimization of key process technology in anode production for reference by enterprises.

Influencing factors of anode quality

1. Raw material quality

1.1. Petroleum coke

Petroleum coke is one of the most important raw materials for the production of prebaked anodes, and its quality is related to the physical and chemical properties of prebaked anodes and the stability of the production process. The quality control of petroleum coke needs to be carried out based on the requirements of environmental protection standards, anode physical and chemical indicators, petroleum coke particle size and cost performance, and it is necessary to focus on controlling ash content, volatile matter, particle size distribution and trace element content. Usually, the ash content of the mixed coke is less than 0.3%, the volatile matter is controlled at 10-11%, the S content is controlled at 2.5-3.5%, and the trace elements such as Na, V, Fe and other trace elements are controlled below 300ppm.

1.2. Coal pitch

Coal tar pitch, as a binder for anode production, requires good wettability and adhesion. The quality of coal tar pitch directly affects the physical and chemical properties and use effect of the anode, and it is necessary to control the softening point, coking value, viscosity, BI, β resin, QI, etc. of coal tar pitch. Usually, the softening point is controlled at 100-110°C, the coking value is >55%, the beta resin is >20%, the QI is controlled at 6-12%, and the batches should be as stable as possible.

2. Process technology

2.1. Calcination process

Calcining is the first process in the production of pre-baked anodes. The calcined coke produced in this process directly affects the physical and chemical properties of pre-baked anodes, and ultimately affects the use of anodes in aluminum electrolytic cells.

The calcination production should meet the requirements of balance, and the inventory of petroleum coke and calcined coke should be balanced, and the crushed particle size of petroleum coke should be balanced. Each kind of petroleum coke is separately stored and weighed to mix petroleum coke to ensure that petroleum coke volatiles, S, Na, V and other trace elements meet the production needs; it is necessary to achieve a balanced temperature control of the fire channel of the retort furnace, to prevent the temperature of the fire channel from fluctuating high and low, resulting in substandard calcination of coke after calcination, and to prevent local high temperature from causing the canister Body burn.

The control direction of the calcination process: the temperature of the second layer is controlled at 1260-1300 ℃, the resistivity of the calcined coke powder is controlled at 470-520 μΩm, and the discharge amount of the calcined coke is not less than 105kg/h·can.

2.2. Forming process

Forming is the core process of pre-baked anode production. The process flow is complex and there are many process control points. In the forming production, the particle size distribution of materials, the ratio of dry material and coal tar pitch, and the kneading and forming temperature will affect the quality of the raw anode.

The molding production must meet the stability requirements, and the equipment must be stable, the material particle size must be stable, and the paste temperature must be stable. Strictly  strengthen the daily inspection and maintenance of equipment, and ensure sufficient spare parts inventory, so as to ensure the operation of the equipment as planned; Try to keep the amount of calcined coke and the negative pressure of the dust collection system stable; To achieve dry mixing , wet mixing, paste cooling, molding temperature deviation as small as possible.

The control direction of the molding process: the purity of ball mill powder: 60-65%, the purity of coarse coke ≥ 85%, the purity of medium coke ≥ 90%, the particle size of residual pole 14-5mm: 25-35%, 5-2mm: 20-30%, Asphalt ratio is reduced to below 14.5%, dry mixing temperature: 95-1O5℃, wet mixing temperature: 160-164℃, molding temperature: 148-155℃.

2.3.  Baking process

Baking is one of the most important heat treatment processes in the production of pre-baked anodes. The baking curve determines the physical and chemical changes of the green anode during the baking process, and directly affects the appearance qualification rate, resistivity, mechanical strength and oxidation resistance of the pre-baked anode. Baking production needs to meet the leak-proof requirements, and the negative pressure and temperature loss should be as small as possible.

Negative pressure loss: It will lead to an increase in the negative pressure at the outlet of the purification system, an increase in power consumption, an increase in the amount of filling material pumped through the fire channel, and an increase in the amount of filler powder drawn into the purification system, which increases the purification load; The difference in the negative pressure of the fire channel increases, making the temperature and The temperature of the product is unbalanced; The difference of volatile matter overflow becomes larger, which makes the volatile matter burn insufficiently, increases the difficulty of temperature adjustment, and affects the quality of the anode product; leads to aggravated deformation of the fire wall and shortens the life of the roaster.

Temperature loss: The temperature loss between 3P and 6P causes the anode temperature to be uneven and the internal structure of the anode to deteriorate; The temperature loss at 1C and 2C results in insufficient holding time for the anode temperature above 1050℃, which affects the quality of the anode product.

The control direction of the baking process: establish a production management mode centered on the baking furnace, man-machine integration, the negative pressure of the fire channel should be controlled within -100Pa as much as possible, the chimney of the purification system will not emit smoke, and the smoke and dust should be controlled at 30mg/m3; the heating rate is the highest controlled at 10°C/h, the volatile matter burns clearly and translucently, preventing the production of tar.

In addition to the quality of raw materials such as petroleum coke and coal tar, the most important factor affecting the quality of prebaked anodes is the influence of calcination, forming and baking technology. As the saying goes, calcination is the foundation, forming is the core, and baking is the guarantee. Taking the influence relationship between each process condition and the physical and chemical indicators of the pre-baked anode as a link, optimizing the physical and chemical indicators of the anode in sub-item, thus to produce high-quality, low-consumption anodes product. Regarding the specific optimization of calcination, molding and roasting technology, we will discuss it in next serial article, please continue to pay attention!