Measures for reducing carbon residue in aluminum electrolysis technology

Foreword

In recent years, with the progress in the production technology of large-scale aluminum reduction cells and the rapid advancement of the management level, the matching of technical parameters of the electrolytic cell production process has become more reasonable, the stability of the electrolytic cell operation has been continuously improved, and the production technical indicators of the electrolytic cell have been continuously improved.

However, the influence of the quality of raw materials on the stability of the operation of the electrolytic cell and the influence of production indicators in the production process of aluminum electrolysis increasingly arouses the attention of the production management personnel, especially the quality of the anode. Anode Carbon block as the heart of aluminum electrolysis, its quality is good or bad, directly affect the conduct of electrolysis and product quality.

If the mass of the carbon block cannot meet the requirements, excessive carbon residue will be generated in the aluminum electrolyte solution, resulting in a series of adverse effects on the aluminum electrolysis process, which will easily cause the electrolyte voltage to rise, resulting in the generation of hot troughs. Caused by the increase in electrolytic consumption, and when the hot trough generated will deteriorate the technical and economic indicators of the production of aluminum electrolysis, while also affecting the life of the electrolyzer, so reducing the production of carbon slag in aluminum electrolysis production into aluminum electrolysis cell production management The important part.

The origin of carbon residue

The anode quality is unstable. The pre-baked carbon block is produced by crushing, calcining, mixing, kneading, and other processes from petroleum coke, asphalt coke, and asphalt. If the raw materials and processes used do not meet the requirements, unqualified carbon blocks will be produced. Such as: low compressive strength, large voidage, large impurities, etc., which lead to oxidation of the anode and carbon particles on the surface of the anode fall off into the electrolyte to form carbon residue, sometimes forming pieces and cracks in the electrolyte erosion and scrubbing Under the formation of carbon residue. The carbon residue caused by the carbon block quality is the main reason for the formation of carbon residue in production.

The unqualified quality of the prebaked anode is the main reason for the production of carbon residue in the electrolytic production process. However, the quality of the prebaked anode is also related to the raw materials used to produce the anode, such as petroleum coke, or forging coke directly purchased on the market, and coal. The influence of trace elements on the quality of anodes in raw materials such as asphalt and residual carbon.

The main raw material for carbon production is petroleum coke. Among them, V, Ni and other impurity elements have a significant effect on air reactivity, while Na has a strong catalytic effect on CO2 reactivity and air reactivity. Secondly, in the electrolytic production process, the residual electrode that is replaced during the production process is cleaned if the electrolyte attached to the surface is not clean, and the anode is brought into the anode during the production of the ingredients. Particularly, the polymer electrolyte has a large amount of Na. In addition, process fluctuations and raw material formulations will also produce unqualified carbon blocks.

The pre-baked anode is cleaned from the binder on the anode surface after being baked out of the roaster, and after entering the electrolytic cell, it gradually falls off into the electrolyte and becomes a carbon residue as the electrolysis reaction proceeds.

Carbon residue produced during electrolytic production

The dregs caused by poor operation quality. The quality of the renewal operation is very important, especially if the anode is not easily handled according to the technical specifications. The new anode cannot realize full-current work at the initial stage of installation. When the anode is installed at a low level, it is easy to form dregs when it is infiltrated in the electrolyte and is washed away. In addition, when the installation of the new pole is too low, the current is smaller than the average value when the current conductance is full. Caused by the current is too large, resulting in strong thermal stress, destruction of the anode strength to form explosion block and dregs. Other work quality, such as improper coverage of alumina insulation material, resulting in anode exposed to produce anodized dregs, especially at the anode corner of the aluminum outlet, the flue end, and the discharge port is more obvious.

The dregs caused by poor cell conditions. When the electrolytic cell condition is abnormal, the working condition of the anode also deteriorates. If the temperature of the cell rises, the oxidation resistance of the anode weakens. At the same time, the temperature of the cell is high, and the carbonization slag is easily enriched in the side furnace, and the electrolytic cell is discharged. The slag function is weakened and it is easy to cause carbon in the electrolyte. 1.4.2 During the electrolysis production process, fine management is not in place, and the quality of the work is rough. After the pole changing, the insulation material cover is not dense, and even if there is a fire or exposure to the air, the high-temperature anode will contact the air and oxidize the dregs.

Because of the long-term high-temperature roasted deformation of the shelling hammerhead, the anode near the hammerhead is accompanied by electrolytes and the hammerhead is enlarged. During the shelling and cutting process, the insulating material on the anode surface is destroyed to expose the anode surface. Contact oxidation with air.

The coke particles used in the installation of the new electrolyzer are loaded, and after the roasting of the electrolyzer is completed, the residual carbon residue is not salvaged.

Erosive peeling inside the cathode carbon. In the process of aluminum electrolysis, erosion peeling and crushing inside the cathode carbon is another source of carbon residue generated in the aluminum electrolytic solution. After the aluminum electrolytic cell is started, due to the penetration of sodium, erosion and erosion of the electrolyte solution and aluminum, the carbon lining of the cathode will soon be exfoliated, and sodium infiltration into the cathode carbon block is the main cause of spalling. The infiltration of sodium causes stress inside the carbon block, which causes the carbon block to expand in volume and become loose and porous, resulting in the spalling of carbon residue.

The second reaction produces free solid carbon. The secondary reaction during the aluminum electrolysis not only reduces the current efficiency, but also brings about the adverse effect of another, namely that the aluminum dissolved in the electrolyte solution reduces the CO2 and CO in the anode gas to C and forms in the electrolyte solution Fine free carbon residue.

There are two kinds of reactions:

The first reaction is that dissolved aluminum in the electrolyte solution reacts with CO2 to produce CO, which in turn reacts with AL to produce C, that is:

2AL(dissolved)+2CO2=AL2O3+3CO(1)

2AL(dissolved)+3CO=AL2O3+3C(2)

The second reaction is that the aluminum in the electrolyte directly reduces CO2 to C,

3AL(dissolved)+3CO2=2AL2O3+3C(3)

The reaction (3) in the above two reactions has a greater effect on the formation of carbon residue in the aluminum electrolyte than reaction (2), but the carbon residue produced by these two reactions is not the generation of carbon residue in the electrolyte solution. main reason.

The effect of carbon residue on the electrolysis process

In the normal production process, with the continuous development of aluminum electrolysis, the carbon anodes are gradually consumed as the production progresses. The carbon slag produced during the normal production of the anodes can be smoothly removed from the electrolyte under reasonable technological conditions. Separation does not have much impact on production, but it is difficult to have such relatively ideal production status in actual production. Therefore, as a production manager, it is necessary to constantly pay attention to changes in the amount of carbon slag in the electrolyte in order to reduce the impact on electrolytic production.

Increase power consumption

The carbon slag in the aluminum electrolytic solution causes the resistance of the electrolyte to increase. As a result, the voltage drop of the electrolyte increases, and the power consumption of the aluminum electrolytic production increases. According to relevant professional reports, when the content of carbon slag in the aluminum electrolyte solution reaches 1% by weight, the electrolyte conductivity is reduced by about 11%. This shows that the adverse effect of carbon slag on the conductivity of the electrolyte is extremely significant. The smaller the size of the carbon slag, the greater the effect on reducing the conductivity of the electrolyte.

Hot trough

If the carbon slag in the electrolyte accumulates to a certain concentration, due to the increase of the specific resistance, the voltage drop of the electrolyte will inevitably increase, so that the electric energy income between the two electrodes of the electrolytic cell will increase additionally, causing the electrolyte to overheat, the temperature of the bath to increase, and the heat to form. groove. After the hot trough is formed, the heat balance of the electrolytic cell is destroyed, the normal technical conditions are affected, and the cathode of the electrolytic cell is damaged, which affects the service life of the trough. In addition, when the hot trough is processed, a large amount of fluoride salts are also consumed. The harm effect is very great.

Cause current consumption

When a large amount of carbon slag floats on the surface of the aluminum electrolyte melt, part of the carbon slag becomes a conductive path for the carbon anode and the side or cathode, and part of the current will directly enter the cathode or the side through the carbon slag, and cannot participate in the electrolytic reaction to form a side portion. Leakage, current consumption, serious side leakage will cause.

Anode long package

Due to the large amount of carbon slag accumulated, it can not be cleared out in time, and it is easy to induce corners or side pockets or teeth of the electrolytic cell, resulting in voltage swing or pressure trough of the electrolytic cell.

Induced anode effect

A large amount of carbon slag floats on the surface of the electrolyte, causing the alumina to not dissolve into the electrolyte in time, thereby inducing an anode effect.

Increase the labor intensity of workers

When the content of carbon residue in the electrolyte is too large, the workers must be organized to salvage. Retrieving the carbon residue not only takes away a large amount of electrolytes and heat, affects the stability of the electrolytic cell, and increases the fluoride consumption. When the carbon residue is salvaged, holes are to be drilled in different parts of the electrolytic cell to facilitate the extraction of carbon residue, and the labor volume of workers is significantly increased.

Measures to reduce carbon residue

Doing a good job of raw material supply management

The main raw materials for the production of carbon coke such as petroleum coke, coal tar pitch, and calcined coke are to be selectively purchased, and used in conjunction with the test results and the oxidation resistance of the carbon block, and suppliers with unstable product quality are taken to stop supplying them. For the mixed residual electrode, the surface of the electrolyte should be completely cleaned to minimize the electrolyte into the anode.

Improve the quality of the anode fabrication process

To improve the quality of carbon anode products, the fundamental is to improve the quality of the anode system process, namely, the quality of petroleum coke calcined, the molding formulation, the kneading temperature of the paste, the quality of the mix, the quality of the green block, and the high-temperature roasting quality of the green block. Standardize various operating procedures and strictly implement technical standards to ensure that the anode surface and internal physical and chemical indicators meet the needs of electrolytic production.

Strengthen the analysis of trace elements

Affect the quality of the carbon anode, lead to affect the use of carbon anode in the electrolytic cell and trace elements of aluminum quality, such as V, Na, S, Ca, Fe, etc. must be strictly controlled, resulting in the electrolysis cell carbon anode residue V, Na Other elements with strong activity should be given more attention and be mixed with ingredients from different origins and quality indicators to achieve the best ratio.

The shape of the anode carbon block was improved, and the bottom surface was free from angular erosion.

The non-cornered carbon block on the lower surface is transformed from a 90-degree right-angled shape to a chamfered or arc-shaped transition angle between the side surface and the bottom surface of the carbon block. Through experiments, it can be observed that the new pole replaced a day ago, the electrical conductivity is poor, but the lower corner turned from a right angle to an arc, indicating that the main reason for arc formation at this time is the erosion of the anode carbon by the electrolyte. The block, and this straight corner of the carbon block all become carbon slag into the electrolyte. The main advantage of non-undercut carbon block is its strong resistance to scouring and can effectively reduce the amount of carbon slag in the tank.

Use high quality anode carbon block

In the previous discussion on the source of carbon slag, the unsatisfactory quality of the carbon block was the main reason for the carbon granules to fall off and produce carbon residue. Therefore, the use of high quality carbon block is an important measure to reduce the production of carbon residue from the electrolyte solution. Therefore, a strict quality inspection is required before the prebaked anode block enters the factory to prevent unqualified anodes from entering the production line.

High-quality cathode carbon block

Like the anode carbon material, the quality of the cathode carbon block has an effect on the degree of flaking of the carbon block. The use of a high-quality cathode side carbon block and a bottom carbon block in the construction of the cathode of the electrolytic cell can effectively withstand and resist the aluminum electrolyte. Erosion and erosion of the solution and the aluminum liquid reduce the flaking of the carbon block and reduce the generation of carbon residue.

Low temperature aluminum electrolytic production process

Since the secondary reaction of aluminum is also a cause of carbon residue generation, the occurrence of secondary reactions is reduced in the electrolytic production process. Actively apply and optimize new processes such as low voltage, low alumina concentration, low molecular ratio, low temperature, and high polar distance to ensure that electrolysis production is performed at a lower temperature and maintain a reasonable degree of superheat, which is beneficial to carbon Separation of slag can reduce the secondary reaction loss of aluminum, thereby reducing the production of carbon residue.

Keep insulation material of proper thickness

Practice has proved that the insulation material is too thin and easy to make the air contact with the anode surface. The anode carbon block exposed to high temperature in the electrolytic cell is in contact with the air surface and the slag is quickly oxidized. The insulation material must be densely covered to avoid contact with air. In addition, when the surface shell is used for covering, the finer the particle size of the surface shell pieces is, the better it is to ensure the denseness of the anode covering.

Maintain proper electrolyte levels

The level of electrolyte is one of the main factors that determine the oxidation residue of carbon block. Electrolyte level is too low, the heat loss of the electrolytic cell is fast, which is not conducive to stable cell conditions, but the electrolyte level is too high, especially exceeding the upper surface of the residual electrode. When the electrolyte liquid flows on the surface of the carbon block, the insulation material on the residual electrode melts. Forming a space will exacerbate the oxidation of the carbon block and increase the amount of carbon residue. Therefore, it is necessary to actually maintain a reasonable electrolyte level.

in conclusion

The pure carbon slag is inevitable in the normal production of aluminum electrolysis. When the content of carbon slag in the electrolysis cell reaches a certain level, it will affect the normal production of the electrolysis cell and bring negative impact.

In the daily production of electrolyzers, measures should be taken to reduce the impact of carbon residue on electrolysis production by improving the quality of anodes, improving the quality of exchanging electrodes, adjusting the composition of electrolytes, controlling the temperature of electrolysis, and making full use of all possible opportunities to salvage carbon slag.