The Aggregate Producer's worst nightmare
Deck blinding is a real challenge for many aggregate operations. Soft, powdery limestone is notorious for bad blinding, but this problem can occur in almost any type of aggregate. Dust-sized particles often deposit themselves on flat surfaces inside a shaker, including the tops of the screening wires. Add a little moisture and the deposits become like hard cement.
The traditional strategies to defeat blinding include the following:
- If possible, increase the top-size particle feed to the deck. Larger particles tend to scour the screening surface.
- Within reason, reduce the size of the wire diameter, if using steel screens. The lighter diameter wire will more effectively pick up the resonance of the screen and improve the screening action. The additional gain in open area also aids screening efficiency. When using light diameter wire, care must be taken to ensure adequate support of the wire cloth.
- Consider the use of long-slot openings (slotted openings). The greater the distance between the wires the more difficult it becomes for the material to bridge the opening. It also sets up a more pronounced "humming" action on the wire, creating a cleansing effect. An inherent danger in slotted openings is their tendency to pass elongated particles (flats). Consult with a Nepean Rubber field representative before proceeding with a changeover to long slots.
- Consider. Nepean Rubber Cast Polyurethane modular clip-on screens are usually a good fix in such a situation.
- For severe blinding challenges, there's a new solution — Nepean Rubber’s Dual Compound flexible flap screens are a good alternative especially when screening finer materials and offers a unique answer to blinding. Square opening screens do not have flexible wires and cannot disrupt the bridging over of the opening that results in blinding. Nepean Rubber Dual Compound flexible flap screens action breaks up this process.
Blinding – Look At Your Middle Deck
An often overlooked cause of blinding on the bottom deck is an inefficient middle deck. If the screening efficiency of the middle deck is not what it could be, then there is likely to be a high percentage of "carryover." Re-processing (re-crushing) generates more fines, so the gradations are likely to be "fining-up" (vs. coursing-up). With an increase of re-grind product in your mix, you're adding more "fine-load" to all your decks, top to bottom. Eventually, your bottom deck can become overburdened with fines, and it will blind over.
Often the best solution is to improve the screening efficiency of your middle deck (assuming a three-deck shaker). If you can create a highly efficient middle deck and minimize the need for secondary processing, you'll go a long way toward eliminating the source of fines in your entire system.