According to the properties of petroleum coke products and the requirements of petroleum coke products for prebaked anodes of the existing refinery production enterprises, the main technical indicators and requirements in petroleum coke are described as follows:
The amount of sulfur in petroleum coke mainly depends on the quality of crude oil.Generally speaking, when the sulfur content range of petroleum coke is relatively low, the anode consumption decreases with the increase of sulfur content, because sulfur increases the coking rate of asphalt and reduces the porosity of asphalt coking. At the same time, sulfur also combines with metal impurities to reduce the catalytic effect of metal impurities and can inhibit carbon dioxide reactivity and air reactivity of carbon anodes.However, if the sulfur content is too high, it will increase the thermal brittleness of the carbon anode, and because sulfur is mainly converted into gas phase in the electrolysis process, it will seriously affect the electrolysis environment and cause great pressure on environmental protection. In addition, iron sulfide film may also be formed on the anode rod, increasing the voltage drop.
With the increasing import of crude oil and the continuous improvement of processing methods in China, the trend of poor quality of petroleum coke is inevitable.According to statistics, the sulfur content of petroleum coke was generally about 1.5 % ten years ago and was relatively stable. Five years ago, it was about 2.0 %, and now it has reached about 4.0 %, some even exceeding 6.0 %.In order to adapt to the change of raw materials, the prebaked anode production enterprises and the electrolytic aluminum industry have carried out a large number of technical reforms and tackling of key problems. According to the investigation of domestic prebaked anode production enterprises, petroleum coke with sulfur content of about 3 % can generally be directly calcined.The situation abroad is about the same. US Rain Cii is a world-famous producer of calcined coke with prebaked anodes. When they import petroleum coke from Sinopec, the sulfur content is controlled at 0.5 – 6.0 %. Calcination is carried out by blending and other technological measures. The sulfur content of calcined coke is controlled at less than 3 %. French aluminum industry, Bahraini aluminum industry and Iran’s related companies also require the sulfur content of calcined coke to be less than 3 %.
In view of the actual situation of domestic petroleum coke and the actual requirements of electrolytic aluminum for prebaked anodes, the aluminum carbon anode industry recommends to control sulfur content below 3.5 %.
The high volatile content of petroleum coke indicates that the non-coking part is carried more.Excessive volatile matter will affect the true density of calcined coke and reduce the yield of calcined coke, but a proper amount of volatile matter will help the calcination of petroleum coke.After the petroleum coke is calcined at high temperature, the volatile matter content is reduced, so different users have different expectations for volatile matter. Combined with the actual situation of enterprise production and user demand, it is suggested that the volatile matter should not be more than 10 % ~ 12 %.
After the combustible part of petroleum coke is completely burned at 850 ℃ under the condition of high temperature and air circulation, the remaining non-combustible mineral impurities ( trace elements ) are called ash. The purpose of ash determination is to identify the content of mineral impurities ( trace elements ) in order to evaluate the quality of coke.
Controlling the ash content, that is, controlling the trace elements, too much ash will certainly affect the anode itself and the quality of the original aluminum.
According to the needs of users and the actual production situation of the enterprise, it is recommended that the ash content should not be greater than 0.3 % ~ 0.5 %.
Coke breeze content
The particle size of petroleum coke has a great influence on the yield, energy consumption and calcined coke. The carbon burning loss of petroleum coke with high content of coke breeze is serious in the calcination process.
Due to the high content of coke breeze, it is easy to cause coking at the discharge opening and furnace blasting, which will easily lead to problems such as early furnace damage, over-burning, blocked discharge valve, loose calcined petroleum coke and easy pulverization, affecting the service life of the calciner. At the same time, the true density, tap density, porosity, strength, resistivity and oxidation performance of calcined coke have been also greatly influenced.Based on the current situation of petroleum coke production and quality in China, experts in the industry were consulted and the coke breeze content ( less than 8 mm ) was controlled within 30 – 50 %.
The organic matter in petroleum coke is cracked at high temperature, and the solid product after escaping gaseous products is fixed carbon.Fixed carbon is the basic component in the preparation of prebaked anode. The fixed carbon content of different petroleum coke is different.The aluminum carbon anode industry usually requires a fixed carbon content of not less than 85 %.
The main sources of water content in petroleum coke are as follows:
First, when coke is discharged from the coke tower, petroleum coke is discharged to the coke pool under the action of hydraulic cutting;
Second, from the safety point of view, spray cooling measures must be taken for petroleum coke that has not been completely cooled after coke discharge;
Third, petroleum coke is basically stacked in the open air in the coke pool and storage yard, and its water content is affected by the environment; Fourth, the structure of petroleum coke is different, and its water holding capacity is also different.According to the present situation of petroleum coke production, stacking and transportation, the general water content shall be implemented according to the contract.
The content of trace elements in petroleum coke directly affects the service life of prebaked anode and the quality and grade of electrolytic aluminum products.Trace elements in petroleum coke mainly include Fe, Ca, V, Na, Si, Ni, P, Al, Pb, etc.Due to the different oil sources of petroleum refiners, the composition and content of trace elements vary greatly.
Some trace elements are brought in from crude oil ( e.g. S, V, etc. ) and are also brought in during crude oil extraction. In addition, some alkali metals and alkaline earth metals are also brought in during processing, and some ash ( e.g. Si, Fe, Ca, etc. ) will also be added during transportation and storage.
Excessive trace element content will directly affect the service life of prebaked anode and the grade of electrolytic aluminum. Ni, V, Ca, Na and other elements have a strong catalytic effect on the anodic oxidation reaction, promoting the selective oxidation of the anode, causing the anode to drop slag and block and increasing the excessive consumption of the anode. Si and Fe mainly affect the quality of raw aluminum. The increase of Si content will increase the hardness and decrease the conductivity of aluminum.
The increase of Fe content will greatly affect the plasticity and corrosion resistance of aluminum alloy. P and Pb mainly affect the current efficiency and environment of electrolyzer, while Al has no obvious influence on anode and electrolytic aluminum.
Shot coke content
The shot coke, also called spherical coke and pellet coke, is relatively hard, dense and non-porous and exists in the form of spherical molten mass.The shot coke is smooth in appearance and inconsistent in internal structure and appearance. Due to the lack of pores on the surface, it is difficult for the binder to penetrate into the interior of the coke when it is kneaded with the binder coal tar pitch, resulting in loose bonding and easy occurrence of internal defects.
In addition, the thermal expansion coefficient of shot coke is high, which can easily cause thermal shock cracking during anode baking. It is not recommended to use a certain proportion of shot coke for prebaked carbon anode .