Schematic Operation Of Filter
Figure 1. Technology, based on fixed vacuum trays, features step-wise movement of filter media.
Madison’s objectives were to expand production and replace the present labor-intensive process with a continuous operation.
Crucial Tests
BHS process engineers began laboratory evaluation of the process. Madison was open to all ideas and formed a team to brainstorm different approaches.
Laboratory testing was done on the BHS pocket leaf filter. The tests were used to evaluate:
1. filter media for good solids retention for visually clear filtrate;
2. filtration flux rate versus cake thickness;
3. wash ratio with respect to cake salt content by measuring % chlorides and conductivity; and
4. cake moisture versus drying time.
BHS conducted several weeks of testing and evaluated both pressure and vacuum filtration based upon the specific characteristics of the solids and slurries. Creativity is the key in these tests to ensure evaluation of all relevant filtration technologies.
The testing led to the following observations:
• Filtrate clarity: The most-appropriate filter cloth is a double-weave 12-micron polypropylene.
• Filtration rate: Vacuum filtration produced the maximum filtration flux rate at a cake thickness of 6 mm.
• Cake washing: Maximum displacement washing was achieved with wash ratios of 2.6:1.
• Cake moisture: Although not a critical parameter because the cake is reslurried, cake moisture is approximately 35%.
Madison agreed with the recommendation of BHS process engineers that continuous-indexing vacuum filtration was the optimum option.
The BHS continuous-indexing vacuum belt filter provides for vacuum filtration, cake washing, pressing and drying of high solids slurries. The technology is based upon fixed vacuum trays, a continuously feeding slurry system and indexing or step-wise movement of the filter media (Figure 1). In practical terms, the belt filter operates similarly to a series of Buchner funnels.
At each indexed belt position, washing and drying efficiencies are maximized with the stopped belt and the mechanism of plug flow for gases and liquids. Cake pressing and squeezing further enhance drying. Finally, the fixed trays allow for the mother liquor and the wash filtrates to be recovered individually and recirculated/recovered/reused for a more efficient operation. The design also can integrate steaming as well as counter-current washing.
To further evaluate this option, testing continued using a larger (0.6 m2) pilot belt filter. These trials achieved the washing results with a lower wash ratio (1.35:1) than in the previous testing and produced higher quality product, one with a residual chloride concentration of less than 0.11%.
