Precipitation of aluminium sulphate in papermaking

In the process of papermaking, due to the instability and change of pH, the aluminium sulfate will change, and a precipitate may be formed, preventing it from fulfilling its role, which will also affect the production quality of the paper, leading to paper breakage and slow production. Let's examine how it becomes sedimentation in papermaking and how to resolve this issue.

When the paper machine operates between pH 3.8 and 4.2, the system has a high content of free inorganic acids, which prevents instability due to pH changes, and in this buffer zone, the pH remains stable until the system becomes superacidic due to the accumulation of excess aluminium sulfate, necessitating equilibration with a higher pH slurry. Although this situation meets the needs of papermakers to control pH and stabilise the pH value of the paper machine's filtered water, it does not achieve good sizing and retention effects. These low-pH systems also experience serious chemical deposition problems, with blanket clogging and increased equipment corrosion.

As a result of excessive addition, a large number of trivalent aluminium ions will appear in the slurry and white water, and when the pH of the system rises, these trivalent aluminium ions will immediately become reactive aluminium polymers and aluminium hydroxide flocs. This phenomenon typically occurs in areas where clean water is used, such as paper machine headboxes, finishing rollers, mesh cleaning, and press felts. Generally, it will lead to the formation of deposition of aluminium sulfate glue on the headbox, facing roller, forming net case and blanket, which affects the effectiveness of use and is not conducive to cleaning. Increased levels can also lead to clogging of orifice plates and press rolls. Clean water is used as spray water and sealing water in certain parts of the paper machine. However, if the hardness of the water is high, dirt and slime consisting of aluminium hydroxide condensates will form and deposit on the pipe wall.

When the above precipitation problem occurs, the following three methods can be employed to address the issue:
1. Increase the pH of the paper machine to reduce the concentration of soluble trivalent aluminium ions.
2. The pH of the water can be partially lowered to avoid pH fluctuations.
3. Add a special chemical separator.