In order to ensure the cleanness of the surface of the aluminum coil, some pretreatment sections should be done before the coil is formed. So as to avoid the residual grease and smooth agent on the surface, which will affect the coating and application. Therefore, aluminum coil should be pretreated in the following aspects before forming.
First of all, you might as well use hot water to wash the aluminum coil. Hot water scrubbing is mainly to clean the residual degreaser on the surface of the substrate, to ensure that these residues can be dissolved, so as to prevent the secondary pollution of the substrate caused by these degreasers. The application methods are mostly immersion and spray brushing. The hardness of the water used should not be very high, otherwise, the minerals in the water will form mineral spots on the surface of the substrate.
Secondly, due to the fast speed of precoating line, the degreaser used in hot alkali degreasing usually has a high concentration. The typical degreaser contains sodium hydroxide (NaOH), sodium carbonate (Na2CO3), sodium silicate (Na2SiO3), phosphate and other components. The hot alkali degreasing process is usually divided into two steps to ensure that the surface of aluminum coil substrate is washed clean, and spray brushing is often used.
Thirdly, it was passivated. Passivation treatment is to form a conversion film on the surface of substrate by pressure spraying, dip coating or roll coating. The sludge generated in the process of using passivation solution will often jam the spray hole, thus affecting the spray efficiency. Although the dip coating method can solve this problem, the amount of passivation solution is large.
Both of the above two methods need water to clean the excess passivation solution in the essential operation, which will lead to the recovery and purification of wastewater. Roller coating is a good passivation construction method, which has the advantages of average coating, economic application and no washing. Chromate / oxide type treatment agent is usually used for aluminum plate, which contains chromate, chromic acid, phosphoric acid and promoters fluoride and molybdate. This kind of treatment agent must be added phosphoric acid, otherwise it can not be used to deal with food and drink aluminum plate.