It is necessary to perform a filtration testing program to determine the filtration rate of the material. You need a representative sample because some parameters (e.g. particle size, percent solids) have an impact on the filtration rate.

As Jorge said, you need to have a filtering test results.  This will give you a filter rate (unit area/unit time for an anticapated product moisture content).  This is your starting point.  Then you look at filter choices and select a filter that can meet your desired product moisture (note product in this case is the filtered discharge which can be a concentrate or a tailings).  Then you check the filter area per time (based on filter operating speed).  This will give you the size of the filter required based on your feed rate.  If the results is at or near the limit of the largest filter than you use 2 (or more).

This is a somewhat iterative process, and you may find that your desired filter type can not meet your desired performance (this is from the manufacturers data).

Thank you Jorge and SmartDog for your response, I am working on sizing my belt filter based on the starting point as you guys suggested. I calculated the filter rate and product moisture from data and assumptions found from other phosphoric acid production plants in the literature. If could, can I ask for the filtration testing program that you guys know so far? I think it may help my result more accurate. Thank you so much for your help.

The simplest is just using the lab vacuum filter such as: https://www.sepor.com/product/laboratory-batch-vacuum-filter/

Which will give you the approximate filter rate, doing a timed filter test:  pour in solution, run and record time to desired dryness, may take several runs.

Beyond that have samples sent to your vendor.