I understand you are looking for a screen. BUT! Upon researching the elements of pumicite it was found to be extremely high on the natural fertilizer scale. Primary ingredient is silica. It is part of basaltic compound formed during volcanic explosions. Post explosion large sheets/islands of pumice float on the ocean. Plants and especially trees will grow mainly because of the sea salt content but also because of the 90% porosity. To buy this rich of a fertilizer would cost at least $100 for a 10 # bag. Since you already have in a liquid form. use it, store it, sell it. You can even begin your own community garden or donate it to a garden. Grow food and donate to local food banks. Just an environmental and cost effective thought. Good luck.
Aquathin designed a project for a similar application whereby an elevated decant tank with integrated 8" mechanical opening at beveled bottom, was able to separate the heavy material and drop into a rail car. The liquid was drained off prior to media drop.
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Rotary Drum screen is the first for me then control the RPM speed including a gab of slit and secondly is normal curve slit (Special) which its shape like sharp triangle. aggasit2012@gmail.com