Help: Seeking effective ways to reduce the content of biuret in finished urea, as well as methods to lower the level of metal ions in finished urea.

The previous issues are covered in most books: residence time, temperature, ammonia partial pressure. More specifically, this includes liquid level control in the stripping tower, liquid level control in the distillation tower, and setting lower limits for operations. The initial temperature in the stripping tower is UDU; the temperature of the liquid leaving the distillation tower, the temperature of the liquid leaving the flash tank, as well as the temperatures in stage 1 and stage 2. The ammonia concentration in the urine is slightly high. Of course, each case must be analyzed on its own. In our case, with a production capacity of 400,000 tons, it was difficult to control the levels of biuret during the initial operation phase – the temperature in stage 1 was around 122 degrees, and that in stage 2 around 133 degrees. If these temperatures were slightly higher, the biuret levels would exceed acceptable limits. Later, it was determined that the equipment used in stages 1 and 2 was not suitable for a production capacity of 400,000 tons; the equipment was too large, causing the urine to be overheated inside the heat exchange tubes. As a result, 10% of the tubes became blocked, and once that problem was resolved, the situation came under control, allowing for higher temperatures to be used

I haven’t understood the latter question: where do the metal ions come from?

The main sources of metal ions in low-quality urea: iron, nickel, chromium, etc. from the erosion and corrosion of stainless steel；

What downstream products are processed, and why is it necessary to control the metal ion content?