Controlling vacuum in a horizontal pan filtering device

A method for continuously filtering a liquid slurry, comprising:

• (a) providing a horizontal pan filtering device having a rotatable filter support for holding a pan filter, the pan filter comprising, in order, a slurry feed section, a first filtrate drainage section having a wet sector and a dry sector, a second filtrate drainage section having a wet sector and a dry sector, and a filter cake discharge section, all sections having a common rotatable screen bottom to prevent solids from passing therethrough;
• (b) depositing the slurry onto the slurry feed section of the pan filter and rotating the filter support until the slurry reaches the wet sector of the first filtrate drainage section;
• (c) drawing a vacuum at the wet sector of the first filtrate drainage section with a first vacuum source to draw the liquid through the filter to form a first filtrate and drained slurry;
• (d) rotating the filter support until the drained slurry reaches the dry sector of the first filtrate drainage section;
• (e) drawing a vacuum at the dry sector of the first filtrate drainage section with a second vacuum source to draw the liquid through the filter and form a second filtrate and a first filter cake;
• (f) rotating the filter support until the first filter cake reaches the wet sector of the second filtrate drainage section where it is washed with a liquid to form a washed first filter cake;
• (g) drawing a vacuum at the wet sector of the second filtrate drainage section with a third vacuum source to draw the liquid through the filter to form a third filtrate and drained washed first filter cake;
• (h) rotating the filter support until the drained washed first filter cake reaches the dry sector of the second filtrate drainage section;
• (i) drawing a vacuum at the dry sector of the second filtrate drainage section with a fourth vacuum source to draw the liquid through the filter to form a fourth filtrate and a second filter cake;
• and
• (j) removing the second filter cake from the filter cake discharge section;
• whereby the first vacuum source and the third vacuum source are capable of generating a higher vacuum than the second vacuum source and the fourth vacuum source.